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13.1: Electromagnetic Waves

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Did you ever wonder how a microwave works? It directs invisible waves of radiation toward the food placed inside of it. The radiation transfers energy to the food, causing it to get warmer. The radiation is in the form of microwaves, which are a type of electromagnetic waves.

What Are Electromagnetic Waves?

Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. Like other waves, electromagnetic waves transfer energy from one place to another. The transfer of energy by electromagnetic waves is called electromagnetic radiation . Electromagnetic waves can transfer energy through matter or across empty space.

Q: How do microwaves transfer energy inside a microwave oven?

A: They transfer energy through the air inside the oven to the food.

May the Force Be With You

A familiar example may help you understand the vibrating electric and magnetic fields that make up electromagnetic waves. Consider a bar magnet, like the one in the Figure below. The magnet exerts magnetic force over an area all around it. This area is called a magnetic field. The field lines in the diagram represent the direction and location of the magnetic force. Because of the field surrounding a magnet, it can exert force on objects without touching them. They just have to be within its magnetic field.

Q: How could you demonstrate that a magnet can exert force on objects without touching them?

A: You could put small objects containing iron, such as paper clips, near a magnet and show that they move toward the magnet.

An electric field is similar to a magnetic field. It is an area of electrical force surrounding a positively or negatively charged particle. You can see electric fields in the following Figure below. Like a magnetic field, an electric field can exert force on objects over a distance without actually touching them.

How an Electromagnetic Wave Begins

An electromagnetic wave begins when an electrically charged particle vibrates. The Figure below shows how this happens. A vibrating charged particle causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field. The two types of vibrating fields combine to create an electromagnetic wave.

How an Electromagnetic Wave Travels

As you can see in the Figure above, the electric and magnetic fields that make up an electromagnetic wave are perpendicular (at right angles) to each other. Both fields are also perpendicular to the direction that the wave travels. Therefore, an electromagnetic wave is a transverse wave. However, unlike a mechanical transverse wave, which can only travel through matter, an electromagnetic transverse wave can travel through empty space. When waves travel through matter, they lose some energy to the matter as they pass through it. But when waves travel through space, no energy is lost. Therefore, electromagnetic waves don’t get weaker as they travel. However, the energy is “diluted” as it travels farther from its source because it spreads out over an ever-larger area.

Electromagnetic Wave Interactions

When electromagnetic waves strike matter, they may interact with it in the same ways that mechanical waves interact with matter. Electromagnetic waves may:

• reflect, or bounce back from a surface;
• refract, or bend when entering a new medium;
• diffract, or spread out around obstacles.

Electromagnetic waves may also be absorbed by matter and converted to other forms of energy. Microwaves are a familiar example. When microwaves strike food in a microwave oven, they are absorbed and converted to thermal energy, which heats the food.

Sources of Electromagnetic Waves

The most important source of electromagnetic waves on Earth is the sun. Electromagnetic waves travel from the sun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources of electromagnetic waves depend on technology. Radio waves, microwaves, and X rays are examples. We use these electromagnetic waves for communications, cooking, medicine, and many other purposes.

Launch the Light Wave simulation below to help you visualize light as a transverse wave moving through the electromagnetic field. Be sure to adjust the wavelength band slider to observe waves of different sizes, such as Radio Waves and X-Rays. There is also an illustration of objects of comparable sizes next to the electromagnetic spectrum to help you imagine the sizes of these invisible light waves.

Interactive Element

• Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. They transfer energy through matter or across space. The transfer of energy by electromagnetic waves is called electromagnetic radiation.
• The electric and magnetic fields of an electromagnetic wave are areas of electric or magnetic force. The fields can exert force over objects at a distance.
• An electromagnetic wave begins when an electrically charged particle vibrates. This causes a vibrating electric field, which in turn creates a vibrating magnetic field. The two vibrating fields together form an electromagnetic wave.
• An electromagnetic wave is a transverse wave that can travel across space as well as through matter. When it travels through space, it doesn’t lose energy to a medium as a mechanical wave does.
• When electromagnetic waves strike matter, they may be reflected, refracted, or diffracted. Or they may be absorbed by matter and converted to other forms of energy.
• The most important source of electromagnetic waves on Earth is the sun. Many other sources of electromagnetic waves depend on technology.
• What is an electromagnetic wave?
• Define electromagnetic radiation.
• Describe the electric and magnetic fields of an electromagnetic wave.
• How does an electromagnetic wave begin? How does it travel?
• Compare and contrast electromagnetic and mechanical transverse waves.
• List three sources of electromagnetic waves on Earth.

Explore More

Watch the electromagnetic wave animation and then answer the questions below.

• Identify the vibrating electric and magnetic fields of the wave.
• Describe the direction in which the wave is traveling.

Additional Resources

Study Guide: Wave Optics Study Guide

Real World Application: Printing with Light, The Incredible Hulk

PLIX: Play, Learn, Interact, eXplore: Energy Levels: Bohr's Atomic Model

Electromagnetic Radiation

Claimed by Zoila de Leon (Fall 2023)

• 1.1 What is a Electromagnetic(EM) Radiation?
• 1.2 General Properties
• 1.3 Problem Solving Method and Equations
• 1.4 Fields Made by Charges and Fields Made by Monopoles
• 2 The EM Spectrum
• 3 Waves and Fields
• 4 A Mathematical Model
• 5 Connectedness: X-Rays
• 6.1 Practice Problems (new section by Zoila)
• 7 References

What is a Electromagnetic(EM) Radiation?

Electromagnetic radiation is a form of energy that is all around us and takes many forms, such as radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

Before 1873, electricity and magnetism were thought to be two different forces. However, in 1873, Scottish Physicist James Maxwell developed his famous theory of electromagnetism. There are four main electro magnetic interactions according to Maxwell:

• The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them
• Magnetic poles come in pairs that attract and repel each other much as electric charges do
• An electric current in a wire produces a magnetic field whose direction depends on the direction of the current
• A moving electric field produces a magnetic field, and vice versa

General Properties

The four Maxwell's Equations provide a complete description of possible spatial patterns of electric and magnetic field in space.

• The Ampere-Maxwell Law
• Gauss's Law
• Faraday's Law

Other than Maxwell's Four equations, there are general properties of all electromagnetic radiation:

• Electromagnetic radiation can travel through empty space. Most other types of waves must travel through some sort of substance. For example, sound waves need either a gas, solid, or liquid to pass through in order to be heard
• The speed of light is always a constant (3 x 10^8 m/s)
• Wavelengths are measured between the distances of either crests or troughs. It is usually characterized by the Greek symbol λ (lambda).

Electromagnetic waves are the self-propagating, mutual oscillation of electric and magnetic fields. The propagation of electromagnetic energy is often referred to as radiation. We can also say that the 'pulse' of these moving fields result in radiation (7).

The equation for propagation is E=cB with c being the speed of light. This equation is derived from combining the two equations E=vB and B=u0e0vE, proving that v is equal to 3e8 meters/second.

Problem Solving Method and Equations

To go about solving/analyzing mathematically an electromagnetic field using Maxwell's equations,this is how we proceed (7)

• Establish the space and time in which the electric and magnetic fields are present
• Check that Maxwell's equations can be applied in the situation above
• Check when the charge accelerates, it produces these fields and therefore radiation
• Show how these fields would interact with matter

The equation of the Radiative Electric Field is: E= 1/(4πe0)*-qa/(c^2r) where a is the acceleration of the particle, c is the speed of light and r is the distance from the original location of the charge to right before the kink. This kink happens on the electric field because of the slight delay when the charge is moved.

Fields Made by Charges and Fields Made by Monopoles

We can differentiate fields made by charges and the ones made by magnetic monopoles. (7) For fields made by charges, when the charge is

• at rest, E=1/r^2 and B=0
• constant speed, E=1/r^2 and B=1/r^2
• accelerating, E=1/r and B=1/r

For fields made by magnetic monopoles, the first point would have E and B switched.

The EM Spectrum

EM spectrum is a span of enormous range of wavelengths and frequencies. The EM spectrum is generally divided into 7 different regions, in order of decreasing wavelength and increasing energy and frequency. It ranges from Gamma rays to Long Radio Waves. Following are the lists of waves:

• Visible Light
• Infrared Rays
• Long radio waves

Although all these waves do different things, there is one thing in common : They all travel in waves.

Infrared radiation can be released as heat or thermal energy. It can also be bounced back, which is called near infrared because of its similarities with visible light energy. Infrared Radiation is most commonly used in remote sensing as infrared sensors collect thermal energy, providing us with weather conditions.

Visible Light is the only part of the electromagnetic spectrum that humans can see with a naked eye. This part of the spectrum includes a range of different colors that all represent a particular wavelength. Rainbows are formed in this way; light passes through matter in which it is absorbed or reflected based on its wavelength. As a result, some colors are reflected more than other, leading to the creation of a rainbow.

Waves and Fields

As we learned in class, electric field is produced when an electron is accelerating. Likewise, EM radiation is created when an atomic particle, like an electron, is accelerated by an electric field. The movement like this produces oscillating electric and magnetic fields, which travel at right angles to each other in a bundle of light energy called a photon. Photons travel in a harmonic wave at the fastest speed possible in the universe.

Electromagnetic waves are formed when an electric field couples with a magnetic field. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

A wavelength (in m) is the distance between two consecutive peaks of a wave. Frequency is the number of waves that form in a given length of time. A wavelength and frequency are interrelated. A short wavelength indicates that the frequency will be higher because one cycle can pass in a shorter amount of time. Likewise, a longer wavelength has a lower frequency because each cycle takes longer to complete.

Waves can be classified according to their nature:

• Mechanical waves
• Electromagnetic waves

Mechanical Waves

Mechanical waves require a medium (matter) to travel through. Examples are sound waves, water waves, ripples in strings or springs.

Water Waves

Sound Waves

Electromagnetic Waves

Electromagnetic waves do not require a medium (matter) to travel through - they can travel through space. Examples are radio waves, visible light, x-rays.

Radio Waves

Visible Lights

A Mathematical Model

The position of the particle is defined by a sine wave:

y = ymaxsin(wt)

Where w is the angular frequency

Amplitude is the distance from the maximum vertical displacement of the wave to the middle of the wave. The Amplitude of the sinusoidal Wave is the height of the peak in the wave measured from the zero line. This measures the magnitude of oscillation of a particular wave. The Amplitude is important because it tells you the intensity or brightness of a wave in comparison with other waves.

The period of the wave is the time between crests in seconds(s).

T = 2pi/w-----(units of seconds)

Frequency is the number of cycles per second, and is expressed as sec-1 or Hertz(Hz). Frequency is directly proportional to energy and can be express as "

E = hv where E is energy, h is Planck's constant ( 6.62607*10^-34J) and v is frequency

f = 1/T f = w/2pi----(Units Hertz)

Wavelength is the distance between crests in meters. Wavelength is equal to the speed of light times frequency. Longer wavelength waves such as radio waves carry low energy; this is why we can listen to the radio without any harmful consequences. Shorter wavelength waves such as x-rays carry higher energy that can be hazardous to our health.

Wavelength and Frequency

The speed of light is the multiplication of the wavelength and frequency.

This diagram shows all properties of waves:

ENERGY FLUX

Is defined by the following equation:

Connectedness: X-Rays

Electromagnetic Radiation while commonly thought of as only including visible light, radio waves, UV waves, and gamma rays; also include X-rays. In 1895, X-rays were initially discovered by William Roentgen, who accidentally fell upon the most important discovery about his life (Figure 1). Roentgen was already working on cathode rays, and because of a fluorescent glow that occurred during his experiments, covered his experimental apparatus with heavy black paper. However, when he did this, he discovered a glow coming from a screen several feet away. Through many more experiments, he discovered that a new type of energy, not cathode rays, were the cause of the glow. He named them “x-rays” and received the 1901 Nobel Prize in Physics. Roentgen never patented his monumental discovery and as a result, numerous researchers set out to find a multitude of uses and capitalize on his work.

Primarily, people could now view objects that were hidden from plain view (i.e. scanners in airports). While X-rays are now used in 100’s of professions (security, chemistry, art galleries), its most important function is to view bones to determine abnormalities in humans. In fact, one of Roentgen’s first x-rays was of his wife’s hand (Figure 2). X-rays fall under the scope of electromagnetic radiation because, like all E.R. waves, it is comprised of photons. X-rays have wavelengths between 0.01 to 10 nanometers and fall between UV and Gamma Waves on the E.R. spectrum (Figure 3). There are two main methods in which an x-ray may be formed. Both require a vacuum-filled tube called an x-ray tube (Figure 4). With an anode on one end and a cathode on the other, an electric current is applied and a high energy electron is projected from the cathode, through the vacuum, and at the anode. In the characteristic x-ray generation approach, the electron from the cathode collides with an inner shell electron on an atom on the anode (Figure 5). Both of these electrons are ejected from the atom and an outer shell electron takes the place of the inner shell one. Because the outer electron must have a lower energy to fill the inner shell hole, it releases a photon with the equivalent energy of the difference between the two energy levels in the atom. This photon is the x-ray that is used to view objects such as bones.

In the Bremsstrahlung x-ray generation method, the electron from the cathode is slowed as it passes the nucleus of an atom at the anode (Figure 6). As it slows and its path is changed, the loses energy (kinetic energy). This energy is also released as a photon which is subsequently called an x-ray. Depending on the voltage and current of the tube and the material of the anode, different types (as in wavelengths and energy) of x-rays can be produced and each one. However, all X-rays will continue to pass through objects until it reaches a material dense that stops it. However, density of the material required depends on the energy of the x-ray. For example, during a medical x-ray, x-rays of a certain energy will pass through soft tissue (skin, organs, etc) but not through bones. The x-rays that pass through the soft tissue will strike the screen and the absence of the x-rays absorbed by the bones will cause a negative space on the screen. The areas where x-rays do not strike will form the image of the bone. While the principles remain the same, x-ray machines today use incredible sophisticated technology to specify the type of x-ray they want and have greatly increased in accuracy since Roentgen’s initial discovery.

• Information and photographs are pulled from references 1 through 5 cited below*

Already, during the Ancient Greek and Roman times, light was studied as the presence of deflection and refraction were noticed. Electromagnetic radiation of wavelengths in the early 19th century. The discovery of infrared radiation is ascribed to astronomer William Herschel, who published his results in 1800 before the Royal Society of London. Herschel used a glass Triangular prism (optics)|prism to refract light from the Sun and detected invisible rays that caused heating beyond the red part of the spectrum, through an increase in the temperature recorded with a thermometer. These "calorific rays" were later termed infrared.

In 1801, Rohann Ritter, discovered the presence of ultraviolet light using salts. It was known that light could darken some silver halides and while doing so, he realized that the region beyond the violet bar (therefore ultraviolet) was more effective in changing the color of the halides. However,in 1864, while summarizing the theories of his time accumulating into his famous set of Maxwell equations, James Clerk Maxwell managed to deduce the speed of light being around 3e8 meters per second. This was instrumental in creating the rest of the spectrum.

In 1887-1888 Physicist Heinrich Hertz not only tried to measure the velocity and frequency of electromagnetic radiation waves at other parts of the known spectrum of the time, but he was also able to prove that Maxwell's findings were correct. He did this on the microwave radiation as well.

The discovery of X-rays occurred in 1895 by Wilhelm Rontgen when his barium platinocyanide detector screen began to glow under the presence of a discharge that passed through a cathode ray tube although the latter was completely covered. Once he determined its possible use, he tried to look at his wife's hand using this new discovery. However x-ray spectroscopy was not institutionalized until later by Karl Manne Siegbahn.

In 1900, Paul Villard discovered Gamma rays although he initially thought that they were particles similar to alpha and beta particles which were emitted during radiation. These 'particles' were later proven to be part of the electromagnetic spectrum.

Practice Problems (new section by Zoila)

1. Elert, Glenn. "X-rays." X-rays – The Physics Hypertextbook. N.p., n.d. Web. 08 Apr. 2017. http://physics.info/x-ray/

2."X-rays." X-rays. N.p., n.d. Web. 08 Apr. 2017. http://www.physics.isu.edu/radinf/xray.htm

3. "Basics of X-ray PhysicsX-ray production." Welcome to Radiology Masterclass. N.p., n.d. Web. 08 Apr. 2017. http://www.radiologymasterclass.co.uk/tutorials/physics/x-ray_physics_production#top_2nd_img

4. "X-Rays." Image: Electromagnetic Spectrum. N.p., n.d. Web. 08 Apr. 2017. https://www.boundless.com/physics/textbooks/boundless-physics-textbook/electromagnetic-waves-23/the-electromagnetic-spectrum-165/x-rays-597-11175/images/electromagnetic-spectrum/

5. "This Month in Physics History." American Physical Society. N.p., n.d. Web. 08 Apr. 2017. https://www.aps.org/publications/apsnews/200111/history.cfm

6. Editors, Spectroscopy. “The Electromagnetic Spectrum: A History.” Spectroscopy Home, 27 Oct. 2017, www.spectroscopyonline.com/electromagnetic-spectrum-history?id=&sk=&date=&&pageID=4.

7. Chabay, Ruth W., and Bruce A. Sherwood. Matter & Interaction II: Electric & Magnetic Interactions, Version 1.2. John Wiley & Sons, 2003.

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23.1: The Electromagnetic Spectrum

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learning objectives

• Compare properties of AM and FM radio waves

Radio Waves

Radio waves are a type of electromagnetic (EM) radiation with wavelengths in the electromagnetic spectrum longer than infrared light. They have have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, radio waves travel at the speed of light. Naturally occurring radio waves are made by lightning or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, communications satellites, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth’s atmosphere—long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight.

Electromagnetic Spectrum : The electromagnetic spectrum, showing the major categories of electromagnetic waves. The range of frequencies and wavelengths is remarkable. The dividing line between some categories is distinct, whereas other categories overlap. Microwaves encompass the high frequency portion of the radio section of the EM spectrum.

Types of Radio Waves and Applications

Radio waves have many uses—the category is divided into many subcategories, including microwaves and electromagnetic waves used for AM and FM radio, cellular telephones and TV.

The lowest commonly encountered radio frequencies are produced by high-voltage AC power transmission lines at frequencies of 50 or 60 Hz. These extremely long wavelength electromagnetic waves (about 6000 km) are one means of energy loss in long-distance power transmission.

Extremely low frequency (ELF) radio waves of about 1 kHz are used to communicate with submerged submarines. The ability of radio waves to penetrate salt water is related to their wavelength (much like ultrasound penetrating tissue)—the longer the wavelength, the farther they penetrate. Since salt water is a good conductor, radio waves are strongly absorbed by it; very long wavelengths are needed to reach a submarine under the surface.

AM Radio Waves

AM radio waves are used to carry commercial radio signals in the frequency range from 540 to 1600 kHz. The abbreviation AM stands for amplitude modulation—the method for placing information on these waves. A carrier wave having the basic frequency of the radio station (for instance, 1530 kHz) is varied or modulated in amplitude by an audio signal. The resulting wave has a constant frequency, but a varying amplitude.

AM Radio : Amplitude modulation for AM radio. (a) A carrier wave at the station’s basic frequency. (b) An audio signal at much lower audible frequencies. (c) The amplitude of the carrier is modulated by the audio signal without changing its basic frequency.

FM Radio Waves

FM radio waves are also used for commercial radio transmission, but in the frequency range of 88 to 108 MHz. FM stands for frequency modulation, another method of carrying information. In this case, a carrier wave having the basic frequency of the radio station (perhaps 105.1 MHz) is modulated in frequency by the audio signal, producing a wave of constant amplitude but varying frequency.

FM Radio : Frequency modulation for FM radio. (a) A carrier wave at the station’s basic frequency. (b) An audio signal at much lower audible frequencies. (c) The frequency of the carrier is modulated by the audio signal without changing its amplitude.

Since audible frequencies range up to 20 kHz (or 0.020 MHz) at most, the frequency of the FM radio wave can vary from the carrier by as much as 0.020 MHz. For this reason, the carrier frequencies of two different radio stations cannot be closer than 0.020 MHz. An FM receiver is tuned to resonate at the carrier frequency and has circuitry that responds to variations in frequency, reproducing the audio information.

FM radio is inherently less subject to noise from stray radio sources than AM radio because amplitudes of waves add noise. Thus, an AM receiver would interpret noise added onto the amplitude of its carrier wave as part of the information. An FM receiver can be fashioned to reject amplitudes other than that of the basic carrier wave and only look for variations in frequency. Thus, since noise produces a variation in amplitude, it is easier to reject noise from FM.

Electromagnetic waves also broadcast television transmission. However, as the waves must carry a great deal of visual as well as audio information, each channel requires a larger range of frequencies than simple radio transmission. TV channels utilize frequencies in the range of 54 to 88 MHz and 174 to 222 MHz (the entire FM radio band lies between channels 88 MHz and 174 MHz). These TV channels are called VHF (very high frequency). Other channels called UHF (ultra high frequency) utilize an even higher frequency range of 470 to 1000 MHz.

The TV video signal is AM, while the TV audio is FM. Note that these frequencies are those of free transmission with the user utilizing an old-fashioned roof antenna. Satellite dishes and cable transmission of TV occurs at significantly higher frequencies, and is rapidly evolving with the use of the high-definition or HD format.

Microwaves are electromagnetic waves with wavelengths ranging from one meter to one millimeter (frequencies between 300 MHz and 300 GHz).

Learning objectives

• Distinguish three ranges of the microwave portion of the electromagnetic spectrum

Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently with frequencies between 300 MHz (0.3 GHz) and 300 GHz. The microwave region of the electromagnetic (EM) spectrum is generally considered to overlap with the highest frequency (shortest wavelength) radio waves. As is the case for all EM waves, microwaves travel in a vacuum at the speed of light. The prefix “micro-” in “microwave” is not meant to suggest a wavelength in the micrometer range. It indicates that microwaves are “small” because have shorter wavelengths as compared to waves used in typical radio broadcasting. The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary. They are used variously between different fields of study (see figure).

Electromagnetic Spectrum : The electromagnetic spectrum, showing the major categories of electromagnetic waves. The range of frequencies and wavelengths is remarkable. The dividing line between some categories is distinct, whereas other categories overlap. Microwaves overlap with the high frequency portion of the radio section of the EM spectrum.

Subcategories of Microwaves

The microwave portion of the radio spectrum can be subdivided into three ranges, listed below from high to low frequencies.

• Extremely high frequency (EHF) is the highest microwave frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered as far infrared light, also referred to as terahertz radiation. This frequency range corresponds to a wavelength range of 10 to 1 millimeter, so it is sometimes called the millimeter band. This band is commonly used in radio astronomy and remote sensing.
• Super high frequency (SHF) is the designation for electromagnetic wave frequencies in the range of 3 GHz to 30 GHz. This band of frequencies is known also as the centimeter band because the wavelengths range from ten to one centimeters. This frequency range is used for most radar transmitters, microwave ovens, wireless LANs, cell phones, satellite communication, microwave radio relay links, and numerous short range terrestrial data links.
• Ultra-high frequency (UHF) designates the microwave frequency range of electromagnetic waves between 300 MHz and 3 GHz, also known as the decimeter band because the wavelengths range from one to ten decimeters, or 10 centimeters to 1 meter. They are used for television broadcasting, cordless phones, walkie-talkies, satellite communication, and numerous other applications.

Sources of Microwaves

Microwaves are the highest-frequency electromagnetic waves that can be produced by currents in macroscopic circuits and devices. Microwaves can also be produced by atoms and molecules—e.g., they are a component of electromagnetic radiation generated by thermal agitation . The thermal motion of atoms and molecules in any object at a temperature above absolute zero causes them to emit and absorb radiation.

Since it is possible to carry more information per unit time on high frequencies, microwaves are quite suitable for communications devices. Most satellite-transmitted information is carried on microwaves, as are land-based long-distance transmissions. A clear line of sight between transmitter and receiver is needed because of the short wavelengths involved.

The sun also emits microwave radiation, although most of it is blocked by Earth’s atmosphere. The Cosmic Microwave Background Radiation (CMBR) is microwave radiation that permeates all of space, and its discovery supports the Big Bang theory of the origin of the universe.

Cosmic Microwave Background : Cosmic background radiation of the Big Bang mapped with increasing resolution.

Devices Employing Microwaves

High-power microwave sources use specialized vacuum tubes to generate microwaves. These devices operate on different principles from low-frequency vacuum tubes, using the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron (used in microwave ovens), klystron, traveling-wave tube (TWT), and gyrotron.

Cavity Magnetron : Cutaway view inside a cavity magnetron as used in a microwave oven.

Microwaves are used by microwave ovens to heat food. Microwaves at a frequency of 2.45 GHz are produced by accelerating electrons. The microwaves then induce an alternating electric field in the oven. Water and some other constituents of food have a slightly negative charge at one end and a slightly positive charge at one end (called polar molecules). The range of microwave frequencies is specially selected so that the polar molecules, in trying to maintain their orientation with the electric field, absorb these energies and increase their temperatures—a process called dielectric heating .

Radar, first developed in World War II, is a common application of microwaves. By detecting and timing microwave echoes, radar systems can determine the distance to objects as diverse as clouds and aircraft. A Doppler shift in the radar echo can determine the speed of a car or the intensity of a rainstorm. Sophisticated radar systems can map the Earth and other planets, with a resolution limited by wavelength. The shorter the wavelength of any probe, the smaller the detail it is possible to observe.

A maser is a device similar to a laser, which amplifies light energy by stimulating photons. The maser, rather than amplifying visible light energy, amplifies the lower-frequency, longer-wavelength microwaves and radio frequency emissions.

Infrared Waves

Infrared (IR) light is EM radiation with wavelengths longer than those of visible light from 0.74 µm to 1 mm (300 GHz to 1 THz).

Skills to Develop

• Distinguish three ranges of the infrared portion of the spectrum, and describe processes of absorption and emission of infrared light by molecules

Infrared (IR) light is electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 0.74 micrometers (µm) to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 300 GHz to 400 THz, and includes most of the thermal radiation emitted by objects near room temperature. Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements.

Subcategories of IR Waves

The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts: It can be divided into three parts:

Atmospheric Transmittance : This is a plot of Earth’s atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation. Most UV wavelengths are absorbed by oxygen and ozone in Earth’s atmosphere. Observations of astronomical UV sources must be done from space.

• Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm) – The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in Earth’s atmosphere absorbs so strongly in this range that it renders the atmosphere in effect opaque. However, there are certain wavelength ranges (“windows”) within the opaque range that allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as “sub-millimeter” in astronomy, reserving far infrared for wavelengths below 200 μm.
• Mid-infrared, from 30 to 120 THz (10 to 2.5 μm) – Hot objects (black-body radiators) can radiate strongly in this range, and human skin at normal body temperature radiates strongly at the lower end of this region. This radiation is absorbed by molecular vibrations, where the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region, since the mid-infrared absorption spectrum of a compound is very specific for that compound.
• Near-infrared, from 120 to 400 THz (2,500 to 750 nm) – Physical processes that are relevant for this range are similar to those for visible light. The highest frequences in this region can be detected directly by some types of photographic film, and by many types of solid state image sensors for infrared photography and videography.

Note that in some fields the boundaries of these categories differ slightly; for example, in astronomy “near-infrared” is considered to extend to 5 μm rather than 2.5 μm.

Heat and Thermal Radiation

Infrared radiation is popularly known as “heat radiation,” but light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun only accounts for 49% of the heating of the Earth, with the rest being caused by visible light that is absorbed then re-radiated at longer wavelengths. Visible light or ultraviolet-emitting lasers can char paper and incandescently hot objects emit visible radiation. Objects at room temperature will emit radiation mostly concentrated in the 8 to 25 µm band, but this is not distinct from the emission of visible light by incandescent objects and ultraviolet by even hotter objects (see sections on black body radiation and Wien’s displacement law).

Heat is energy in transient form that flows due to temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, radiation can propagate through a vacuum.

The concept of emissivity is important in understanding the infrared emissions of objects. This is a property of a surface which describes how its thermal emissions deviate from the ideal of a black body. To further explain, two objects at the same physical temperature will not “appear” the same temperature in an infrared image if they have differing emissivities.

Sources of IR Waves

As stated above, while infrared radiation is commonly referred to as heat radiation, only objects emitting with a certain range of temperatures and emissivities will produce most of their electromagnetic emission in the infrared part of the spectrum. However, this is the case for most objects and environments humans encounter in our daily lives. Humans, their surroundings, and the Earth itself emit most of their thermal radiation at wavelengths near 10 microns, the boundary between mid and far infrared according to the delineation above. The range of wavelengths most relevant to thermally emitting objects on earth is often called the thermal infrared. Many astronomical objects emit detectable amounts of IR radiation at non-thermal wavelengths.

Infrared radiation can be used to remotely determine the temperature of objects (if the emissivity is known). This is termed thermography, mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to the massively reduced production costs.

Thermography : A thermographic image of a dog

Applications of IR waves extend to heating, communication, meteorology, spectroscopy, astronomy, biological and medical science, and even the analysis of works of art.

Visible Light

Visible light is the portion of the electromagnetic spectrum that is visible to the human eye, ranging from roughly 390 to 750 nm.

• Distinguish six ranges of the visible spectrum

Visible light, as called the visible spectrum, is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is often simply referred to as “light”. A typical human eye will respond to wavelengths from about 390 to 750 nm (0.39 to 0.75 µm). In terms of frequency, this corresponds to a band in the vicinity of 400–790 THz. A light-adapted eye generally has its maximum sensitivity at around 555 nm (540 THz), in the green region of the optical spectrum. The spectrum does not, however, contain all the colors that the human eyes and brain can distinguish. Unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths.

Visible light is produced by vibrations and rotations of atoms and molecules, as well as by electronic transitions within atoms and molecules. The receivers or detectors of light largely utilize electronic transitions. We say the atoms and molecules are excited when they absorb and relax when they emit through electronic transitions.

Visible Spectrum : A small part of the electromagnetic spectrum that includes its visible components. The divisions between infrared, visible, and ultraviolet are not perfectly distinct, nor are those between the seven rainbow colors.

The figure above shows this part of the spectrum, together with the colors associated with particular pure wavelengths. Red light has the lowest frequencies and longest wavelengths, while violet has the highest frequencies and shortest wavelengths. Blackbody radiation from the Sun peaks in the visible part of the spectrum but is more intense in the red than in the violet, making the Sun yellowish in appearance.

Colors that can be produced by visible light of a narrow band of wavelengths (monochromaticlight) are called pure spectral colors. Quantitatively, the regions of the visible spectrum encompassing each spectral color can be delineated roughly as:

• red – 620 to 750 nm (400-484 THz)

Note that each color can come in many shades, since the spectrum is continuous. The human eye is insensitive to electromagnetic radiation outside this range. By definition any images presented with data recorded from wavelengths other than those in the visible part of the spectrum (such as IR images of humans or animals or astronomical X-ray images) are necessarily in false color.

Visible Light and Earth’s Atmosphere

Visible wavelengths pass through the “optical window”, the region of the electromagnetic spectrum which allows wavelengths to pass largely unattenuated through the Earth’s atmosphere (see opacity plot in. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, and so the midday sky appears blue.

The optical window is also called the visible window because it overlaps the human visible response spectrum. This is not coincidental as humanity’s ancestors evolved vision that could make use of the most plentiful wavelengths of light.The near infrared (NIR) window lies just out of the human vision, as well as the Medium Wavelength IR (MWIR) window and the Long Wavelength or Far Infrared (LWIR or FIR) window though other animals may experience them.

A consequence of the existence of the optical window in Earth’s atmosphere is the relatively balmy temperature conditions on Earth’s surface. The Sun’s luminosity function peaks in the visible range and light in that range is able to travel to the surface of the planet unattenuated due to the optical window. This allows visible light to heat the surface. The surface of the planet then emits energy primarily in infrared wavelengths, which has much greater difficulty escaping (and thus causing the planet to cool) due to the opacity of the atmosphere in the infrared. Earth’s surface would be much cooler without this effect.

Photosynthesis

Plants, like animals, have evolved to utilize and respond to parts of the electromagnetic spectrum they are embedded in. Plants (and many bacteria) convert the light energy captured from the Sun into chemical energy that can be used to fuel the organism’s activities. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is vital for all aerobic life on Earth (such as humans and animals). The portion of the EM spectrum used by photosynthesic organisms is called the photosynthetically active region (PAR) and corresponds to solar radiation between 400 and 700 nm, substantially overlapping with the range of human vision. This is again not coincidental; the light in this range is the most plentiful to organisms on the surface of Earth because the Sun emits about half of its luminosity in this wavelength range and it is allowed to pass freely through the optical windows in Earth’s atmosphere.

Ultraviolet Light

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light in the range 10 nm to 400 nm.

• Identify wavelength range characteristic for ultraviolet light and its biological effects

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, that is, in the range 10 nm to 400 nm, corresponding to photon energies from 3 eV to 124 eV (1 eV = 1.6e -19 J; EM radiation with frequencies higher than those of visible light are often expressed in terms of energy rather than frequency). It is so-named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet. These frequencies are invisible to humans, but visible to a number of insects and birds.

UV light is found in sunlight (where it constitutes about 10% of the energy in vacuum) and is emitted by electric arcs and specialized lights such as black lights. It can cause chemical reactions, and causes many substances to glow or fluoresce. Most ultraviolet is classified as non-ionizing radiation. The higher energies of the ultraviolet spectrum from wavelengths about 10 nm to 120 nm (‘extreme’ ultraviolet) are ionizing, but this type of ultraviolet in sunlight is blocked by normal molecular oxygen (O 2 ) in air, and does not reach the ground. However, the entire spectrum of ultraviolet radiation has some of the biological features of ionizing radiation, in doing far more damage to many molecules in biological systems than is accounted for by simple heating effects (an example is sunburn). These properties derive from the ultraviolet photon’s power to alter chemical bonds in molecules, even without having enough energy to ionize atoms.

Although ultraviolet radiation is invisible to the human eye, most people are aware of the effects of UV on the skin, called suntan and sunburn. In addition to short wave UV blocked by oxygen, a great deal (>97%) of mid-range ultraviolet (almost all UV above 280 nm and most up to 315 nm) is blocked by the ozone layer, and like ionizing short wave UV, would cause much damage to living organisms if it penetrated the atmosphere. After atmospheric filtering, only about 3% of the total energy of sunlight at the zenith is ultraviolet, and this fraction decreases at other sun angles. Much of it is near-ultraviolet that does not cause sunburn, but is still capable of causing long term skin damage and cancer. An even smaller fraction of ultraviolet that reaches the ground is responsible for sunburn and also the formation of vitamin D (peak production occurring between 295 and 297 nm) in all organisms that make this vitamin (including humans). The UV spectrum thus has many effects, both beneficial and damaging, to human health.

Atmospheric Transmittance : This is a plot of Earth’s atmospheric opacity (opposite of transmittance) to various wavelengths of electromagnetic radiation, including visible light. Visible light passes relatively unimpeded through the atmosphere in the “optical window.” Most UV wavelengths are absorbed by oxygen and ozone in Earth’s atmosphere. Observations of astronomical UV sources must be done from space.

Subcategories of UV Light

Solar UV radiation is commonly subdivided into three regions: UV-A (320–400 nm), UV-B (290–320 nm), and UV-C (220–290 nm), ranked from long to shorter wavelengths (from smaller to larger energies). Most UV-B and all UV-C is absorbed by ozone (O 3 ) molecules in the upper atmosphere. Consequently, 99% of the solar UV radiation reaching the Earth’s surface is UV-A.

There are other schemes for dividing UV into different categories, another common one is: near-ultraviolet (NUV – 300-400 nm), middle ultraviolet (MUV – 200-300 nm), far ultraviolet (FUV – 200-122 nm), and extreme ultraviolet (EUV- 121-10 nm).

Harmful Effects

An overexposure to UVB radiation can cause sunburn and some forms of skin cancer. In humans, prolonged exposure to solar UV radiation may result in acute and chronic health effects on the skin, eye, and immune system. Moreover, UVC can cause adverse effects that can variously be mutagenic or carcinogenic.

DNA UV Mutation : Ultraviolet photons harm the DNA molecules of living organisms in different ways. In one common damage event, adjacent thymine bases bond with each other, instead of across the “ladder. ” This “thymine dimer” makes a bulge, and the distorted DNA molecule does not function properly.

The International Agency for Research on Cancer of the World Health Organization has classified all categories and wavelengths of ultraviolet radiation as a Group 1 carcinogen. This is the highest level designation for carcinogens and means that “there is enough evidence to conclude that it can cause cancer in humans. ”

Beneficial Effects

UVB exposure induces the production of vitamin D in the skin. The majority of positive health effects are related to this vitamin. It has regulatory roles in calcium metabolism (which is vital for normal functioning of the nervous system, as well as for bone growth and maintenance of bone density), immunity, cell proliferation, insulin secretion, and blood pressure.

X-rays are electromagnetic waves with wavelengths in the range of 0.01 to 10 nanometers and energies in the range of 100 eV to 100 keV.

• Distinguish two categories of X-rays and their biological effects

X-rays are electromagnetic waves with wavelengths in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×10 16 Hz to 3×10 19 Hz) and energies in the range 100 eV to 100 keV. They are shorter in wavelength than UV rays and longer than gamma rays. In many languages, X-radiation is called Röntgen radiation, after Wilhelm Röntgen, who is usually credited as its discoverer, and who had named it X-radiation to signify an unknown type of radiation.

Properties and Applications

X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds. This makes it a type of ionizing radiation and thereby harmful to living tissue. A very high radiation dose over a short amount of time causes radiation sickness, while lower doses can give an increased risk of radiation-induced cancer. In medical imaging this increased cancer risk is generally greatly outweighed by the benefits of the examination. The ionizing capability of X-rays can be utilized in cancer treatment to kill malignant cells using radiation therapy. It is also used for material characterization using X-ray spectroscopy.

X-Ray Spectrum and Applications : X-rays are part of the electromagnetic spectrum, with wavelengths shorter than those of visible light. Different applications use different parts of the X-ray spectrum.

X-rays with photon energies above 5 to 10 keV (below 0.2-0.1 nm wavelength), are called hard X-rays, while those with lower energy are called soft X-rays. Due to their penetrating ability, hard X-rays are widely used to image the inside of objects (e.g., in medical radiography and airport security). As a result, the term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. Since the wavelength of hard X-rays are similar to the size of atoms, they are also useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air and the attenuation length of 600 eV (~2 nm) X-rays in water is less than 1 micrometer.

In medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the radiation dose without contributing to the image. Hence, a thin metal sheet, often of aluminum, called an X-ray filter, is usually placed over the window of the X-ray tube, absorbing the low energy part in the spectrum. This is called hardening the beam since it shifts the center of the spectrum towards higher energy (or harder) X-rays.

Distinction Between X-Rays and Gamma Rays

The distinction between X-rays and gamma rays is somewhat arbitrary. The most frequent method of distinguishing between X- and gamma radiation is the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10 −11 m, defined as gamma rays. The electromagnetic radiation emitted by X-ray tubes generally has a longer wavelength than the radiation emitted by radioactive nuclei. Historically, therefore, an alternative means of distinguishing between the two types of radiation has been by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus. There is overlap between the wavelength bands of photons emitted by electrons outside the nucleus, and photons emitted by the nucleus. Like all electromagnetic radiation, the properties of X-rays (or gamma rays) depend only on their wavelength and polarization.

Gamma rays are very high frequency electromagnetic waves usually emitted from radioactive decay with frequencies greater than 10 19 Hz.

• Identify wavelength range characteristic for gamma rays, noting their biological effects and distinguishing them from gamma rays

Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency and therefore high energy. Gamma rays typically have frequencies above 10 exahertz (or >10 19 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometers (less than the diameter of an atom). However, this is not a hard and fast definition, but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay are defined as gamma rays no matter what their energy, so that there is no lower limit to gamma energy derived from radioactive decay. Gamma decay commonly produces energies of a few hundred keV, and almost always less than 10 MeV.

Gamma Decay : Illustration of an emission of a gamma ray (γ) from an atomic nucleus

Gamma rays are ionizing radiation and are thus biologically hazardous. They are classically produced by the decay from high energy states of atomic nuclei, a process called gamma decay, but are also created by other processes. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium during its gamma decay. Villard’s radiation was named “gamma rays” by Ernest Rutherford in 1903.

Gamma Ray Sources

Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes such as potassium-40, and also as a secondary radiation from various atmospheric interactions with cosmic ray particles. Some rare terrestrial natural sources that produce gamma rays that are not of a nuclear origin, are lightning strikes and terrestrial gamma-ray flashes, which produce high energy emissions from natural high-energy voltages. Gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth’s atmosphere and must be detected by spacecraft. Notable artificial sources of gamma rays include fission such as occurs in nuclear reactors, and high energy physics experiments, such as neutral pion decay and nuclear fusion.

Gamma Rays vs. X-Rays

Gamma rays have characteristics identical to X-rays of the same frequency—they differ only in source. At higher frequencies, γ rays are more penetrating and more damaging to living tissue. They have many of the same uses as X-rays, including cancer therapy. Gamma radiation from radioactive materials is used in nuclear medicine.

The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubes almost invariably had a longer wavelength than the radiation (gamma rays) emitted by radioactive nuclei. Older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10 −11 m, defined as gamma rays. However, with artificial sources now able to duplicate any electromagnetic radiation that originates in the nucleus, as well as far higher energies, the wavelengths characteristic of radioactive gamma ray sources vs. other types, now completely overlap. Thus, gamma rays are now usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

Exceptions to this convention occur in astronomy, where gamma decay is seen in the afterglow of certain supernovas, but other high energy processes known to involve other than radioactive decay are still classed as sources of gamma radiation. A notable example is extremely powerful bursts of high-energy radiation normally referred to as long duration gamma-ray bursts, which produce gamma rays by a mechanism not compatible with radioactive decay. These bursts of gamma rays, thought to be due to the collapse of stars called hypernovas, are the most powerful events so far discovered in the cosmos. Astrophysical processes are the only sources for very high energy gamma rays (~100 MeV).

Gamma Ray Sky Map : This is an image of the entire sky in 100 MeV or greater gamma rays as seen by the EGRET instrument aboard the CGRO spacecraft. Bright spots within the galactic plane are pulsars (spinning neutron stars with strong magnetic fields), while those above and below the plane are thought to be quasars (galaxies with supermassive black holes actively accreting matter).

Health Effects

All ionizing radiation causes similar damage at a cellular level, but because rays of alpha particles and beta particles are relatively non-penetrating, external exposure to them causes only localized damage (e.g., radiation burns to the skin). Gamma rays and neutrons are more penetrating, causing diffuse damage throughout the body (e.g., radiation sickness, cell’s DNA damage, cell death due to damaged DNA, increasing incidence of cancer) rather than burns. External radiation exposure should also be distinguished from internal exposure, due to ingested or inhaled radioactive substances, which, depending on the substance’s chemical nature, can produce both diffuse and localized internal damage. The most biological damaging forms of gamma radiation occur at energies between 3 and 10 MeV.

• The lowest frequency portion of the electromagnetic spectrum is designated as “radio,” generally considered to have wavelengths within 1 millimeter to 100 kilometers or frequencies within 300 GHz to 3 kHz.
• There is a wide range of subcategories contained within radio including AM and FM radio. Radio waves can be generated by natural sources such as lightning or astronomical phenomena; or by artificial sources such as broadcast radio towers, cell phones, satellites and radar.
• AM radio waves are used to carry commercial radio signals in the frequency range from 540 to 1600 kHz. The abbreviation AM stands for amplitude modulation—the method for placing information on these waves. AM waves have constant frequency, but a varying amplitude.
• FM radio waves are also used for commercial radio transmission in the frequency range of 88 to 108 MHz. FM stands for frequency modulation, which produces a wave of constant amplitude but varying frequency.
• The microwave region of the electromagnetic (EM) spectrum is generally considered to overlap with the highest frequency (shortest wavelength ) radio waves.
• The prefix “micro-” in “microwave” is not meant to suggest a wavelength in the micrometer range. It indicates that microwaves are “small” compared to waves used in typical radio broadcasting in that they have shorter wavelengths.
• The microwave portion of the electromagnetic spectrum can be subdivided into three ranges listed below from high to low frequencies: extremely high frequency (30 to 300 GHz), super high frequency (3 to 30 GHz), and ultra-high frequency (300 MHz to 3 GHz).
• Microwave sources include artificial devices such as circuits, transmission towers, radar, masers, and microwave ovens, as well as natural sources such as the Sun and the Cosmic Microwave Background.
• Microwaves can also be produced by atoms and molecules. They are, for example, a component of electromagnetic radiation generated by thermal agitation. The thermal motion of atoms and molecules in any object at a temperature above absolute zero causes them to emit and absorb radiation.
• Infrared light includes most of the thermal radiation emitted by objects near room temperature. Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements.
• The infrared portion of the spectrum can be divided into three regions in wavelength: far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm); mid-infrared, from 30 to 120 THz (10 to 2.5 μm); and near-infrared, from 120 to 400 THz (2,500 to 750 nm).
• Infrared radiation is popularly known as ” heat radiation,” but light and electromagnetic waves of any frequency will heat surfaces that absorb them.
• The concept of emissivity is important in understanding the infrared emissions of objects. This is a property of a surface which describes how its thermal emissions deviate from the ideal of a black body.
• Infrared radiation can be used to remotely determine the temperature of objects (if the emissivity is known). This is termed thermography, mainly used in military and industrial applications.
• Visible light is produced by vibrations and rotations of atoms and molecules, as well as by electronic transitions within atoms and molecules. We say the atoms and molecules are excited when they absorb and relax when they emit through electronic transitions.
• This figure shows the visible part of the spectrum, together with the colors associated with particular pure wavelengths. Red light has the lowest frequencies and longest wavelengths, while violet has the highest frequencies and shortest wavelengths.
• Colors that can be produced by visible light of a narrow band of wavelengths are called pure spectral colors. They can be delineated roughly in wavelength as: violet (380-450 nm), blue (450-495 nm), green (495-570 nm), yellow (570-590 nm), orange (590-620 nm), and red (620 to 750 nm).
• Visible wavelengths pass through the optical window, the Earth’s atmosphere allows this region of the electromagnetic spectrum to pass through largely unattenuated (see opacity plot in.
• The portion of the EM spectrum used by photosynthesic organisms is called the photosynthetically active region (PAR) and corresponds to solar radiation between 400 and 700 nm, substantially overlapping with the range of human vision.
• Ultraviolet light gets its name because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the color violet.
• Most UV is non- ionizing radiation, though UV with higher energies (10-120 nm) is ionizing. All UV can have harmful effects on biological matter (such as causing cancers) with the highest energies causing the most damage.
• The danger posed by lower energy UV radiation is derived from the ultraviolet photon ‘s power to alter chemical bonds in molecules, even without having enough energy to ionize atoms.
• Solar UV radiation is commonly subdivided into three regions: UV-A (320–400 nm), UV-B (290–320 nm), and UV-C (220–290 nm), ranked from long to shorter wavelengths (from smaller to larger energies).
• Most UV-B and all UV-C is absorbed by ozone (O 3 ) molecules in the upper atmosphere. Consequently, 99% of the solar UV radiation reaching the Earth’s surface is UV-A.
• X-rays have shorter wavelengths (higher energy ) than UV waves and, generally, longer wavelengths (lower energy) than gamma rays. Sometimes X-rays are called Röntgen radiation, after Wilhelm Röntgen, who is usually credited as their discoverer.
• Because X-rays have very high energy they are known as ionizing radiation and can harm living tissue. A very high radiation dose over a short amount of time causes radiation sickness, while lower doses can give an increased risk of radiation-induced cancer.
• Lower doses of X-ray radiation can be very effectively used in medical radiography and X-ray spectroscopy. In the case of medical radiography, the benefits of using X-rays for examination far outweighs the risk.
• X-rays are broken up into broad two categories: hard X-rays with energies above 5-10 keV (below 0.2-0.1 nm wavelength) and soft X-rays with energies 100 eV – 5 keV (10 – 0.1 nm wavelength). Hard X-rays are more useful for radiography because they pass through tissue.
• The distinction between X-rays and gamma rays is somewhat arbitrary and there is substantial overlap at the high energy boundary. However, in general they are distinguished by their source, with gamma rays originating from the nucleus and X-rays from the electrons in the atom.
• Gamma rays are the highest energy EM radiation and typically have energies greater than 100 keV, frequencies greater than 10 19 Hz, and wavelengths less than 10 picometers.
• Gamma rays from radioactive decay are defined as gamma rays no matter what their energy, so that there is no lower limit to gamma energy derived from radioactive decay. Gamma decay commonly produces energies of a few hundred keV, and almost always less than 10 MeV.
• Gamma rays have characteristics identical to X-rays of the same frequency—they differ only in source. Gamma rays are usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
• Natural sources of gamma rays include gamma decay from naturally occurring radioisotopes such as potassium-40, and also as a secondary radiation from atmospheric interactions with cosmic ray particles. Exotic astrophysical processes will also produce gamma rays.
• Gamma rays are ionizing radiation and are thus biologically hazardous. The most biological damaging forms of gamma radiation occur at energies between 3 and 10 MeV.
• AM radio waves : Waves used to carry commercial radio signals between 540 and 1600 kHz. Information is carried by amplitude variation, while the frequency remains constant.
• FM radio waves : Waves used to carry commercial radio signals between 88 and 108 MHz. Information is carried by frequency modulation, while the signal amplitude remains constant.
• radio waves : Designates a portion of the electromagnetic spectrum having frequencies ranging from 300 GHz to 3 kHz, or equivalently, wavelengths from 1 millimeter to 100 kilometers.
• terahertz radiation : Electromagnetic waves with frequencies around one terahertz.
• thermal agitation : The thermal motion of atoms and molecules in any object at a temperature above absolute zero, causing them to emit and absorb radiation.
• radar : A method of detecting distant objects and determining their position, velocity, or other characteristics by analysis of sent radio waves (usually microwaves) reflected from their surfaces.
• emissivity : The energy-emitting propensity of a surface, usually measured at a specific wavelength.
• thermography : Any of several techniques for the remote measurement of the temperature variations of a body, especially by creating images produced by infrared radiation.
• thermal radiation : The electromagnetic radiation emitted from a body as a consequence of its temperature; increasing the temperature of the body increases the amount of radiation produced, and shifts it to shorter wavelengths (higher frequencies) in a manner explained only by quantum mechanics.
• spectral color : a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths. Every wavelength of light is perceived as a spectral color, in a continuous spectrum; the colors of sufficiently close wavelengths are indistinguishable.
• optical window : the optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground. The window runs from around 300 nanometers (ultraviolet-C) at the short end up into the range the eye can use, roughly 400-700 nm and continues up through the visual infrared to around 1100 nm, which is thermal infrared.
• visible light : the part of the electromagnetic spectrum, between infrared and ultraviolet, that is visible to the human eye
• ozone layer : A region of the stratosphere, between 15 and 30 kilometres in altitude, containing a relatively high concentration of ozone; it absorbs most solar ultraviolet radiation.
• ionizing radiation : high-energy radiation that is capable of causing ionization in substances through which it passes; also includes high-energy particles
• non-ionizing radiation : Radiation that does not cause atmospheric ionization; electrically neutral radiation.
• X-ray spectroscopy : The use of an X-ray spectrometer for chemical analysis.
• x-ray crystallography : A technique in which the patterns formed by the diffraction of X-rays on passing through a crystalline substance yield information on the lattice structure of the crystal, and the molecular structure of the substance.
• radiograph : An image, often a photographic negative, produced by radiation other than normal light; especially an X-ray photograph.
• gamma ray : A very high frequency (and therefore very high energy) electromagnetic radiation emitted as a consequence of radioactivity.
• gamma decay : A nuclear reaction with the emission of a gamma ray.

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• Radio frequency. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Radio_frequency . License : CC BY-SA: Attribution-ShareAlike
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• OpenStax College, College Physics. April 28, 2014. Provided by : OpenStax CNX. Located at : http://cnx.org/content/m42444/latest/?collection=col11406/1.7 . License : CC BY: Attribution
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• Microwaves. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Microwaves . License : Public Domain: No Known Copyright
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• thermography. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/thermography . License : CC BY-SA: Attribution-ShareAlike
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• Gamma rays. Provided by : Wikipedia. Located at : http://en.Wikipedia.org/wiki/Gamma_rays . License : Public Domain: No Known Copyright

The Nature of Waves

A wave is a disturbance that propagates through a medium. There are three words in that definition that may need unpacking: disturbance, propagate, and medium.

• a change in a kinematic variable like position, velocity, or acceleration;
• a change in an intensive property like pressure, density, or temperature;
• a change in field strength like electric field strength, magnetic field strength, or gravitational field strength.
• To propagate , in the sense used in this definition, is to transmit the influence of something in a particular direction. Synonyms for propagate include spread, transmit, communicate, and broadcast. The noun form of the word is propagation .
• A medium is the substance through which a wave can propagate. Water is the medium of ocean waves. Air is the medium through which we hear sound waves. The electric and magnetic fields are the medium of light. People are the medium of a stadium wave. The Earth is the medium of seismic waves (earthquake waves). Cell membranes are the medium of nerve impulses. Transmission lines are the medium of alternating current electric power. Medium is the singuar form of the noun. Media is the plural form (although mediums is prefered by some people).

Let's list a few key examples of wave phenomena and then connect them to this definition. The first example that comes to mind when most people hear the word wave are the kinds of waves that one sees on the surface of a body of water: deep water waves in the ocean or ripples in a puddle. The most important kinds of waves for humans are the waves we use to sense the world around us: sound and light.

Imagine a calm pool. The surface is flat and smooth. Drop a rock into it. Kerploop. The surface is now disturbed. It is higher than normal in some places and lower than normal in others. The disturbed water at the point of impact disturbs the water next to it, which in turn disturbs the water next to it, which disturbs the water next to it, and so on. The disturbance spreads outward, transmits, or propagates. The medium through which this disturbance propagates is the surface of the water.

Imagine a quiet room. The air inside is still. Drop a book onto a table in that room. Thwap. The air between the book and the table is squeezed out in a fraction of a second. The air pressure in that rapidly decreasing gap rises above normal and then rebounds. The rise and fall of pressure is like the rise and fall of the surface of the pool in the previous example. The air under the book bumps the air on the edges of the book, which bumps the air next to it, which bumps the air next to it, and so on. The medium through which this disturbance propagates is the air.

Those were the easy examples. Water waves and sound waves are examples of mechanical waves — waves that propagate through a material medium. Light is not so easy to understand as a wave, which is why there are multiple sections of this book devoted to it. Still, I am going to try to describe it briefly.

Imagine a dark cavern, deep within the Earth. The electric and magnetic fields inside are relatively static and unchanging. Strike a match. Skeerach. The atoms of carbon in the wood of the matchstick combine with the atoms of oxygen in the air releasing heat. The heat agitates the atoms of the combustion products resulting in the phenomenon known as fire. The electrons bound to the rapidly vibrating atoms disturb the electric and magnetic fields in the space surrounding them. These fields are "elastic" in a sense. A wiggle in the fields in one place causes a wiggle in the fields nearby, which causes a wiggle in the fields nearby, and so on. These wiggles eventually make it to your eye, which you perceive as light. The electric and magnetic fields that fill all of space are the medium.

essential property

Waves transfer energy, momentum, and information, but not mass.

A naive description of a wave is that it has something to do with motion. But the motion of a wave on the water is not the same as the motion of the water from a hose. When waves move over the surface of the ocean, where does the ocean go? Nowhere. When waves reach the shore, does the water accumulate into great heaps? No. The water moves in and out, and the ocean stays behind. Even when huge tsunamis strike, the wall of water deposited on the land eventually drains back into the sea. In this case, no net transfer of mass has occurred.

Compare this to the water from a hose. The water comes pouring out the open end and stays where it lands forming a puddle or drains away to some other location. It most certainly does not jump back into the hose. In this case, mass has been transferred from one location to another.

Any sensible person who owns waterfront property should be familiar with the word erosion. Ocean waves (or waves on the Great Lakes for that matter) break on the shore, beating the rock and soil into submission and pulling it away. This material will never return. (If there were no plate tectonic forces lifting the land up in some places or volcanoes creating new land in others, the Earth would be covered in a global sea of uniform depth.) A force ( F ) has been exerted and mass has been displaced ( ∆ s ). Work has been done ( W  =  F ∆ s ). The ability to do work is one definition of energy ( W  = ∆ E ). Thus waves transfer energy.

Sticking with the example of ocean waves, anyone who surfs knows that waves transfer momentum. I have less to say on this subject.

Sound and light are the two primary examples of the way we gather information around us as humans. We have specialized sensory organs called ears and eyes for doing just that.

Here's a list of some phenomena or activities that satisfy the definition of a wave given above.

• including hock waves
• Radio waves
• Visible light
• Ultraviolet
• Tsunamis (tidal waves)
• Ripples (capillary waves)
• P waves (primary waves, pressure waves)
• S waves (secondary waves, shear waves)
• R waves (Rayleigh waves, ground roll)
• L waves (Love waves)
• A fluttering flag
• Snapping a sheet when making a bed
• Nerve impulses
• Peristalsis
• Heart contractions
• Snakes, eels, etc
• Worms, slugs, etc.
• Centipedes, millipedes, caterpillars, etc.
• Plucking, bowing, or striking a guitar, violin, or piano string
• Casting loops when fly fishing
• Cracking a whip
• Gravitational waves (as described in general relativity, not to be confused with gravity waves in water)
• Matter waves (quantum mechanical waves, de Broglie waves)
• Dominoes (as a show, not as a game)
• Stadium wave (Mexican wave, the wave)
• Newton's cradle (paid link)

not examples

Just because the word wave is used doesn't mean the thing being described is a wave in the sense used in this book.

• Waving as a signal to get someone's attention or to greet them or to say goodbye is not a wave. It does not propagate in a direction. Just because you wave at me does not mean that I have to start waving followed by a person behind me and the person behind them and the person behind them.
• A permanent wave set in a person's hair is not a wave. The term is almost an oxymoron. Nothing's moving if a thing is permanent. Also, you getting a wave set in your hair does not result in the people nearest you getting one followed by the people next to them getting one, and so on, until the whole globe is filled with wavy haired people.
• Wheat, or any other tall grass, is sometimes said to wave when gusts of wind pass over it. That bulk flow of matter is not a wave and neither is the response of the wheat.
• A heat wave is a meteorological term referring to a prolonged period of unusually high temperature. This definition has no connection to a phenomenon that propagates. Just because it's hot for a long period in one location does not imply that the heat wave will propagate to another location. It's actually sort of the opposite. A heat wave is often a region where the hot air is "stuck". The opposite of a heat wave could be called a cold wave, but it's usually described a cold snap (at least in the dialect of English I'm used to hearing). This should indicate that neither one of these phenomena is really a wave. Sometimes infrared radiation is described colloquially as heat waves, but that's not the right term. The same goes for rising thermals in the desert. That shimmering effect sometimes seen on the horizon is turbulent air of different density streaming upward and not a wave. Waves do not transfer mass.
• A crime wave is like a heat wave, but for crime. Since heat waves aren't waves, neither are crime waves.
• Making waves, meaning to stir up trouble, is not an example of a wave — and don't you disagree with me you trouble maker.

Classification of waves

Waves can be classified according to the medium through which they propagate.

by the type of disturbance

Waves can be classified according to the type of disturbance — meaning its relative direction or shap. There is a lot that can be said about this organizational scheme. I'm starting this part of this section with a quick summary in table form followed by a rather detailed follow up.

transverse waves

A transverse wave is one in which the direction of the disturbance is perpendicular to the direction of propagation. The word transverse describes something pointing in a sideways or lateral direction. As dynamic phenomena, waves are better represented with animations than with static images. Click on the static image below to see a transverse wave in action.

A cartoon representation of this kind of wave is your classic wiggly line. People with a bit of math knowledge will tell you they drew a sine curve. Those with a little bit more math knowledge will say they drew a sine or cosine curve.

The high parts on a curve like this are called crests . The low parts are called troughs . Since directions like up and down don't always make sense for waves, what the parts really represent are the maximal changes. The points labeled crests are points corresponding to a maximal increase of the changing quantity in a whatever direction is decided to be positive. The points labeled troughs are the points corresponding to the maximal change in the opposite direction.

Pronouncing words ending in -ough in English is often a mystery. The word trough rhymes with off. A trough is what one uses to provide food and water to livestock and other domesticated animals — typically a long, narrow open container that an animal would dip its head down into. The word crest rhymes with best. A crest is something at the top of something. Many birds, usually male birds, have crests. Hills and mountains are are sometimes said to have crests. The crest on a men's sports jacket or a school uniform gets its name from the heraldic crests that were originally worn on knight's helmets above the visor. Crests are up high. Troughs are down low.

The most important example of a transverse wave for humans is light. Most of what I am about to say in the following bullets will really be discussed elsewhere in this book.

• Showing that light is a just wave was not easy before the 20th century. Now that we have easy access to lasers in the 21st century, it's less of a problem. Light can be made to interfere with itself and produce a pattern of what are called interference fringes . A laser and two or more closely spaced openings are all that is needed. The iridescence seen when gasoline is spilled on water, in some insects like scarab beetles and butterflies, and in pearls and the shells of mollusks is caused by thin film interference . Observing this wave behavior of light requires no special technology.
• Showing that light is a transverse wave was was also not easy before the 20th century. Now that we have easy access to polarized sunglasses and polarized electronic displays in the 21st century, it's less of a problem. Light can be shown to exhibit polarization . Try looking at certain electronic displays with polarized sunglasses. If the orientation of the sunglasses is perpendicular to the orientation of the display, the display will look dark (or really screwed up). If the orientation is parallel, the display will look normal (or closer to normal than when they were perpendicular).

Lots of musical instruments make use of transverse waves to generate their characteristic sound.

• The source of the sound that comes out of violins, guitars, pianos and other chordophones is the side to side motion of a nylon or metal string (or in the olden days, dried animal intestines). The parts of the string vibrate side to side, but the wave travels along the length of the string. These two directions are perpendicular, which makes the waves transverse. The vibrations of the string are also transmitted into the wooded bodies or sound boards of these instruments. These flat wooden parts are driven to flex in and out, but the energy propagates across the surface. The two directions here are also perpendicular.
• The source of the sound that comes out of drum heads, kazoos or other membranophones is the in and out vibration of some flat, membrane-like structure. The waves produced by striking, stroking, or humming into these devices generates waves that crisscross the object. Once again, the disturbance is perpendicular to the propagation.
• Most percussion instruments that are not drums are classified as idiophones . Cymbals, triangles, and xylophones produce sound by the vibration of the entire object or a piece that is not a string, membrane, or column of air. The waves set up in many of these instruments are transverse, but because the class is so large and varied there is probably an exception out there somewhere.

Some animals propel themselves by sending transverse waves down the length of their bodies.

• Fish use a variety of means for getting around but long, thin, tubular fish like eels, lamprey, and dogfish are what comes to mind when I think about locomotion by transverse waves. A wave of side to side motion starts at the head and propagates backward along the spine. This propels the fish forward.
• Snakes also have several mechanisms for propelling themselves. The one that is considered most "normal" is called lateral undulation and has the classic look of a transverse wave in a one-dimensional medium. Much like the fish described above, a wave starts at the head as side to side motion and propagates backward down the length of the snake. A fancier kind of locomotion that relies on transverse waves is called sidewinding and the snakes that use it live in sandy deserts or slippery mud flats — anywhere getting a good grip on the ground is difficult. It's still an example of a transverse wave, but it propels the snake diagonally instead of forward relative to its body axis.

longitudinal waves

• pressure, compression, density
• aerophones - vibrating columns of air (horns, whistles, organ pipes )
• primary (P) pressure
• invertebrates (worms)
• traffic jams
• density waves in spiral galaxies generate the arms
• compressions (a.k.a. condensation): the pressed part, the greatest positive pressure change, a region where the medium is under compression
• rarefactions (a.k.a. dilations): the stretched part, the lowest negative pressure change, a region where the medium is under tension

complex waves

classed by orientation of change

• P rimary (surface, compression, P ressure)
• S econdary (transverse, S hear), can't propagate through liquids
• L ove, ( L ateral shear)
• R ayleigh, (elliptical, plate waves, ground R oll), something like ocean waves, but elliptical instead of circular

torsional waves

By duration.

classified by duration

by appearance

Waves can be classified according to what they appear to be doing.

Now look at these pretty, moving pictures.

Introduction to the Electromagnetic Spectrum

What is Electromagnetic energy?

Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. The human eye can only detect only a small portion of this spectrum called visible light. A radio detects a different portion of the spectrum, and an x-ray machine uses yet another portion. NASA's scientific instruments use the full range of the electromagnetic spectrum to study the Earth, the solar system, and the universe beyond.

When you tune your radio, watch TV, send a text message, or pop popcorn in a microwave oven, you are using electromagnetic energy. You depend on this energy every hour of every day. Without it, the world you know could not exist.

Our Protective Atmosphere

Our Sun is a source of energy across the full spectrum, and its electromagnetic radiation bombards our atmosphere constantly. However, the Earth's atmosphere protects us from exposure to a range of higher energy waves that can be harmful to life. Gamma rays, x-rays, and some ultraviolet waves are "ionizing," meaning these waves have such a high energy that they can knock electrons out of atoms. Exposure to these high-energy waves can alter atoms and molecules and cause damage to cells in organic matter. These changes to cells can sometimes be helpful, as when radiation is used to kill cancer cells, and other times not, as when we get sunburned.

Atmospheric Windows

Electromagnetic radiation is reflected or absorbed mainly by several gases in the Earth's atmosphere, among the most important being water vapor, carbon dioxide, and ozone. Some radiation, such as visible light, largely passes (is transmitted) through the atmosphere. These regions of the spectrum with wavelengths that can pass through the atmosphere are referred to as "atmospheric windows." Some microwaves can even pass through clouds, which make them the best wavelength for transmitting satellite communication signals.

While our atmosphere is essential to protecting life on Earth and keeping the planet habitable, it is not very helpful when it comes to studying sources of high-energy radiation in space. Instruments have to be positioned above Earth's energy-absorbing atmosphere to "see" higher energy and even some lower energy light sources such as quasars.

Next: Anatomy of an Electromagnetic Wave

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Introduction to the Electromagnetic Spectrum. Retrieved [insert date - e.g. August 10, 2016] , from NASA Science website: http://science.nasa.gov/ems/01_intro

Science Mission Directorate. "Introduction to the Electromagnetic Spectrum" NASA Science . 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/01_intro

Discover More Topics From NASA

James Webb Space Telescope

Perseverance Rover

Parker Solar Probe

16.1 Traveling Waves

Learning objectives.

By the end of this section, you will be able to:

• Describe the basic characteristics of wave motion
• Define the terms wavelength, amplitude, period, frequency, and wave speed
• Explain the difference between longitudinal and transverse waves, and give examples of each type
• List the different types of waves

We saw in Oscillations that oscillatory motion is an important type of behavior that can be used to model a wide range of physical phenomena. Oscillatory motion is also important because oscillations can generate waves, which are of fundamental importance in physics. Many of the terms and equations we studied in the chapter on oscillations apply equally well to wave motion ( Figure 16.2 ).

Types of Waves

A wave is a disturbance that propagates, or moves from the place it was created. There are three basic types of waves: mechanical waves, electromagnetic waves, and matter waves.

Basic mechanical wave s are governed by Newton’s laws and require a medium. A medium is the substance mechanical waves propagate through, and the medium produces an elastic restoring force when it is deformed. Mechanical waves transfer energy and momentum, without transferring mass. Some examples of mechanical waves are water waves, sound waves, and seismic waves. The medium for water waves is water; for sound waves, the medium is usually air. (Sound waves can travel in other media as well; we will look at that in more detail in Sound .) For surface water waves, the disturbance occurs on the surface of the water, perhaps created by a rock thrown into a pond or by a swimmer splashing the surface repeatedly. For sound waves, the disturbance is a change in air pressure, perhaps created by the oscillating cone inside a speaker or a vibrating tuning fork. In both cases, the disturbance is the oscillation of the molecules of the fluid. In mechanical waves, energy and momentum transfer with the motion of the wave, whereas the mass oscillates around an equilibrium point. (We discuss this in Energy and Power of a Wave .) Earthquakes generate seismic waves from several types of disturbances, including the disturbance of Earth’s surface and pressure disturbances under the surface. Seismic waves travel through the solids and liquids that form Earth. In this chapter, we focus on mechanical waves.

Electromagnetic waves are associated with oscillations in electric and magnetic fields and do not require a medium. Examples include gamma rays, X-rays, ultraviolet waves, visible light, infrared waves, microwaves, and radio waves. Electromagnetic waves can travel through a vacuum at the speed of light, v = c = 2.99792458 × 10 8 m/s . v = c = 2.99792458 × 10 8 m/s . For example, light from distant stars travels through the vacuum of space and reaches Earth. Electromagnetic waves have some characteristics that are similar to mechanical waves; they are covered in more detail in Electromagnetic Waves .

Matter waves are a central part of the branch of physics known as quantum mechanics. These waves are associated with protons, electrons, neutrons, and other fundamental particles found in nature. The theory that all types of matter have wave-like properties was first proposed by Louis de Broglie in 1924. Matter waves are discussed in Photons and Matter Waves .

Mechanical Waves

Mechanical waves exhibit characteristics common to all waves, such as amplitude, wavelength, period, frequency, and energy. All wave characteristics can be described by a small set of underlying principles.

The simplest mechanical waves repeat themselves for several cycles and are associated with simple harmonic motion. These simple harmonic waves can be modeled using some combination of sine and cosine functions. For example, consider the simplified surface water wave that moves across the surface of water as illustrated in Figure 16.3 . Unlike complex ocean waves, in surface water waves, the medium, in this case water, moves vertically, oscillating up and down, whereas the disturbance of the wave moves horizontally through the medium. In Figure 16.3 , the waves causes a seagull to move up and down in simple harmonic motion as the wave crests and troughs (peaks and valleys) pass under the bird. The crest is the highest point of the wave, and the trough is the lowest part of the wave. The time for one complete oscillation of the up-and-down motion is the wave’s period T . The wave’s frequency is the number of waves that pass through a point per unit time and is equal to f = 1 / T . f = 1 / T . The period can be expressed using any convenient unit of time but is usually measured in seconds; frequency is usually measured in hertz (Hz), where 1 Hz = 1 s −1 . 1 Hz = 1 s −1 .

The length of the wave is called the wavelength and is represented by the Greek letter lambda ( λ ) ( λ ) , which is measured in any convenient unit of length, such as a centimeter or meter. The wavelength can be measured between any two similar points along the medium that have the same height and the same slope. In Figure 16.3 , the wavelength is shown measured between two crests. As stated above, the period of the wave is equal to the time for one oscillation, but it is also equal to the time for one wavelength to pass through a point along the wave’s path.

The amplitude of the wave ( A ) is a measure of the maximum displacement of the medium from its equilibrium position. In the figure, the equilibrium position is indicated by the dotted line, which is the height of the water if there were no waves moving through it. In this case, the wave is symmetrical, the crest of the wave is a distance + A + A above the equilibrium position, and the trough is a distance − A − A below the equilibrium position. The units for the amplitude can be centimeters or meters, or any convenient unit of distance.

The water wave in the figure moves through the medium with a propagation velocity v → . v → . The magnitude of the wave velocity is the distance the wave travels in a given time, which is one wavelength in the time of one period, and the wave speed is the magnitude of wave velocity. In equation form, this is

This fundamental relationship holds for all types of waves. For water waves, v is the speed of a surface wave; for sound, v is the speed of sound; and for visible light, v is the speed of light.

Transverse and Longitudinal Waves

We have seen that a simple mechanical wave consists of a periodic disturbance that propagates from one place to another through a medium. In Figure 16.4 (a), the wave propagates in the horizontal direction, whereas the medium is disturbed in the vertical direction. Such a wave is called a transverse wave . In a transverse wave, the wave may propagate in any direction, but the disturbance of the medium is perpendicular to the direction of propagation. In contrast, in a longitudinal wave or compressional wave, the disturbance is parallel to the direction of propagation. Figure 16.4 (b) shows an example of a longitudinal wave. The size of the disturbance is its amplitude A and is completely independent of the speed of propagation v .

A simple graphical representation of a section of the spring shown in Figure 16.4 (b) is shown in Figure 16.5 . Figure 16.5 (a) shows the equilibrium position of the spring before any waves move down it. A point on the spring is marked with a blue dot. Figure 16.5 (b) through (g) show snapshots of the spring taken one-quarter of a period apart, sometime after the end of` the spring is oscillated back and forth in the x -direction at a constant frequency. The disturbance of the wave is seen as the compressions and the expansions of the spring. Note that the blue dot oscillates around its equilibrium position a distance A , as the longitudinal wave moves in the positive x -direction with a constant speed. The distance A is the amplitude of the wave. The y -position of the dot does not change as the wave moves through the spring. The wavelength of the wave is measured in part (d). The wavelength depends on the speed of the wave and the frequency of the driving force.

Waves may be transverse, longitudinal, or a combination of the two. Examples of transverse waves are the waves on stringed instruments or surface waves on water, such as ripples moving on a pond. Sound waves in air and water are longitudinal. With sound waves, the disturbances are periodic variations in pressure that are transmitted in fluids. Fluids do not have appreciable shear strength, and for this reason, the sound waves in them are longitudinal waves. Sound in solids can have both longitudinal and transverse components, such as those in a seismic wave. Earthquakes generate seismic waves under Earth’s surface with both longitudinal and transverse components (called compressional or P-waves and shear or S-waves, respectively). The components of seismic waves have important individual characteristics—they propagate at different speeds, for example. Earthquakes also have surface waves that are similar to surface waves on water. Ocean waves also have both transverse and longitudinal components.

Example 16.1

Wave on a string.

• The speed of the wave can be derived by dividing the distance traveled by the time.
• The period of the wave is the inverse of the frequency of the driving force.
• The wavelength can be found from the speed and the period v = λ / T . v = λ / T .
• The first wave traveled 30.00 m in 6.00 s: v = 30.00 m 6.00 s = 5.00 m s . v = 30.00 m 6.00 s = 5.00 m s .
• The period is equal to the inverse of the frequency: T = 1 f = 1 2.00 s −1 = 0.50 s . T = 1 f = 1 2.00 s −1 = 0.50 s .
• The wavelength is equal to the velocity times the period: λ = v T = 5.00 m s ( 0.50 s ) = 2.50 m . λ = v T = 5.00 m s ( 0.50 s ) = 2.50 m .

Significance

Check your understanding 16.1.

When a guitar string is plucked, the guitar string oscillates as a result of waves moving through the string. The vibrations of the string cause the air molecules to oscillate, forming sound waves. The frequency of the sound waves is equal to the frequency of the vibrating string. Is the wavelength of the sound wave always equal to the wavelength of the waves on the string?

Example 16.2

Characteristics of a wave.

• The amplitude and wavelength can be determined from the graph.
• Since the velocity is constant, the velocity of the wave can be found by dividing the distance traveled by the wave by the time it took the wave to travel the distance.
• The period can be found from v = λ T v = λ T and the frequency from f = 1 T . f = 1 T .
• The distance the wave traveled from time t = 0.00 s t = 0.00 s to time t = 3.00 s t = 3.00 s can be seen in the graph. Consider the red arrow, which shows the distance the crest has moved in 3 s. The distance is 8.00 cm − 2.00 cm = 6.00 cm . 8.00 cm − 2.00 cm = 6.00 cm . The velocity is v = Δ x Δ t = 8.00 cm − 2.00 cm 3.00 s − 0.00 s = 2.00 cm/s . v = Δ x Δ t = 8.00 cm − 2.00 cm 3.00 s − 0.00 s = 2.00 cm/s .
• The period is T = λ v = 8.00 cm 2.00 cm/s = 4.00 s T = λ v = 8.00 cm 2.00 cm/s = 4.00 s and the frequency is f = 1 T = 1 4.00 s = 0.25 Hz . f = 1 T = 1 4.00 s = 0.25 Hz .

Check Your Understanding 16.2

The propagation velocity of a transverse or longitudinal mechanical wave may be constant as the wave disturbance moves through the medium. Consider a transverse mechanical wave: Is the velocity of the medium also constant?

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Electromagnetic waves are a form of radiation that travel though the universe. They are formed when an electric field (Fig. 1 red arrows) couples with a magnetic field (Fig.1 blue arrows).

Both electricity and magnetism can be static (respectively, what holds a balloon to the wall or a refrigerator magnet to metal), but when they change or move together, they make waves. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

Unlike sound waves, which must travel through matter by bumping molecules into each other like dominoes (and thus can not travel through a vacuum like space), electromagnetic waves do not need molecules to travel. They can travel through air, solid objects, and even space, making them very useful for a lot of technologies.

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Science News Explores

Explainer: understanding waves and wavelengths.

Waves transfer energy, not matter, from one place to another

The swells of water in the ocean, the sunlight shining down and the sound of the crashing water ⎯ all are types of waves.

JNEphotos/iStock/Getty Images Plus

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By Jennifer Look

March 5, 2020 at 6:40 am

Waves appear in many different forms. Seismic waves shake the ground during earthquakes. Light waves travel across the universe, allowing us to see distant stars. And every sound we hear is a wave. So what do all these different waves have in common?

A wave is a disturbance that moves energy from one place to another. Only energy —  not matter — is transferred as a wave moves.

The substance that a wave moves through is called the medium . That medium moves back and forth repeatedly, returning to its original position. But the wave travels along the medium. It does not stay in one place.

Imagine holding one end of a piece of rope. If you shake it up and down, you create a wave, with the rope as your medium. When your hand moves up, you create a high point, or crest. As your hand moves down, you create a low point, or trough (TRAWF). The piece of rope touching your hand doesn’t move away from your hand. But the crests and troughs do move away from your hand as the wave travels along the rope. 

The same thing happens in other waves. If you jump in a puddle, your foot pushes on the water in one spot. This starts a small wave. The water that your foot hits moves outward, pushing on the water nearby. This movement creates empty space near your foot, pulling water back inwards. The water oscillates, moving back and forth, creating crests and troughs. The wave then ripples across the puddle. The water that splashes at the edge is a different bit of water than where your foot made contact. The energy from your jump moved across the puddle, but the matter (the molecules of water) only rocked back and forth.

Light, or electromagnetic radiation, also can be described as a wave. The energy of light travels through a medium called an electromagnetic field. This field exists everywhere in the universe. It oscillates when energy disturbs it, just like the rope moves up and down as someone shakes it. Unlike a wave in water or a sound wave in air, light waves don’t need a physical substance to travel through. They can cross empty space because their medium does not involve physical matter. 

Scientists use several properties to measure and describe all these types of waves. Wavelength is the distance from one point on a wave to an identical point on the next, such as from crest to crest or from trough to trough. Waves can come in a wide range of lengths. The wavelength for an ocean wave might be around 120 meters (394 feet). But a typical microwave oven generates waves just 0.12 meter (5 inches) long. Visible light and some other types of electromagnetic radiation have far tinier wavelengths.  

Frequency describes how many waves pass one point during one second. The units for frequency are hertz . Traveling through the air, a music note with a frequency of 261.6 hertz (middle C) pushes air molecules back and forth 261.6 times every second. 

Frequency and wavelength are related to the amount of energy a wave has. For example, when making waves on a rope, it takes more energy to make a higher frequency wave. Moving your hand up and down 10 times per second (10 hertz) requires more energy than moving your hand only once per second (1 hertz). And those 10 hertz waves on the rope have a shorter wavelength than ones at 1 hertz. 

Many researchers rely on the properties and behavior of waves for their work. That includes astronomers, geologists and sound engineers. For example, scientists can use tools that capture reflected sound, light or radio waves to map places or objects. 

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What radio waves can travel through and what they cannot.

When Guglielmo Marconi began research into long-range radio in the late 19th century, the scientific community was skeptical. That he succeeded in transmitting radio signals more than 2000km met with much astonishment. What he had unwittingly done was discover the ionosphere, a layer in the atmosphere that is ionized to a degree by the sun’s ultraviolet rays. 

This affects radio waves and reflects them to earth, which bounces them upwards again. This repeated sequence enables us to transmit radio waves around the world. So, if radio waves can’t penetrate the ionosphere, what else will hinder them? Let’s look at what radio waves consist of, and then we’ll talk about their ability or otherwise to pass through certain materials.

Radio Waves Explained 

Developments in radio technology have brought us a long way in the past 100 years and more, but how do radio waves work? Before Marconi’s discovery, scientists believed that radio signals could only travel a straight line, as they were known to do in free space. Hence, there was a necessity for tall towers from which to transmit signals. A 100m high tower, for example, could send a signal up to around 30km, the limit set by the curvature of the earth. Marconi showed that radio waves could be bent and bounced by the ionosphere.

Radio waves form a small part of what is known as the electromagnetic spectrum (EMS). This consists not just of radio waves but also other forms of electromagnetic radiation such as gamma rays, microwaves, visible light, and more. Radio waves have among the longest wavelengths on the EMS. Imagine a rope held between two people. Move the rope up and down at one end, and a waveform moves across the rope.

This is what a radio wave would look like if we could see it. The wavelength is the distance between two ‘peaks’ in the wave. We also have to consider the frequency, which the number of waves that pass a given point in a set time. This is vital to understand so that different transmissions do not interfere with each other. 

Television signals are radio waves, but they are broadcast in a different frequency range to the radio.

What has this to do with the materials that radio waves can and cannot pass through? Let’s look more closely at this. 

What Can’t Radio Waves Pass-Through?

We’ve already seen that radio waves do not penetrate the ionosphere. We should explain that this refers to radio waves of up to 40MHz, the highest frequency bounced off the ionosphere. Why do radio waves bounce back?

As we mentioned, the ionosphere is partly ionized. 

This is because the UV light from the sun causes electrons to shake free from atoms, leaving a large proportion of free electrons hanging about in the ionosphere. When a radio wave hits the ionosphere, the energy created by the free electrons, as a result, causes the radio waves to be repelled or reflected. 

So, we know that radio waves up to 40MHz won’t pass through an ionized layer in the atmosphere. Remember that we explained the perceived limitations of radio transmissions before Marconi’s discovery. A line of sight was necessary, without obstructions, for successful transmission.

In some radio transmissions, interference can be caused by the waves being blocked by materials that they cannot penetrate. While radio waves can move through wood, bricks, and concrete to a certain extent, these materials can still cause interference. This is because radio waves meeting such objects can be reflected from buildings and other large structures, just like the ionosphere. This explains why the reception of radios and other radio frequency devices such as smartphones vary between locations.

For the record, as it covers a large bandwidth necessarily, television signals need to be above 40MHz so they cannot be bounced off the ionosphere, hence the tall towers in high locations used for TV transmissions. 

Radio waves will also be adversely affected by metal and water, which they cannot pass through. That is because water and metal are both electrical conductors. Like the ionosphere, metal and water contain many free electrons, which will vibrate when a radio wave hits the surface, and the wave will bounce back. 

To summarize, we now know that radio waves are affected by free electrons in electrical conductors such as metals and water and cannot pass through these materials. 

We have discovered that they can pass through non-conducting materials quite well but are losing propagation power with reflecting surfaces such as water, metal, and other materials. Furthermore, worldwide radio transmission is made possible by the radio waves being reflected by the ionosphere and the earth in a continuous sequence, allowing for transmissions to be made across great distances. 

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What Do Radio Waves Tell Us about the Universe?

Radio astronomy began in 1933 when an engineer named Karl Jansky accidentally discovered that radio waves come not just from inventions we create but also from natural stuff in space. Since then, astronomers have built better and better telescopes to find these cosmic radio waves and learn more about where they come from and what they can tell us about the universe. While scientists can learn a lot from the visible light they detect with regular telescopes, they can detect different objects and events – such as black holes, forming stars, planets in the process of being born, dying stars, and more – using radio telescopes. Together, telescopes that can see different kinds of waves – from radio waves to visible light waves to gamma rays – give a more complete picture of the universe than any one type of telescope can on its own.

When you look up at the night sky, you see the bright lights of stars. If you live in a dark place far from cities, you can see thousands of them. But the individual dots you see are all nearby stars. About 100 billion more stars, exist just in our galaxy, which is called the Milky Way. Beyond the Milky Way, astronomers think that about 100 billion more galaxies (each with their own 100 billion stars) exist. Almost all of these stars are invisible to your eyes, which cannot see the dim light from distant stars. Your eyes miss other things, too. The visible light that your eyes can see is only a tiny portion of what astronomers call the “ electromagnetic spectrum ,” the whole range of different light waves that exists. The electromagnetic spectrum also includes gamma rays, X-rays, ultraviolet radiation, infrared radiation, microwaves, and radio waves. Because human eyes can only see visible light, we have to build special telescopes to pick up the rest of that “spectrum” – and then turn them into pictures and graphs that we can see.

What is a Radio Wave?

Light is made up of tiny particles called “ photons .” Photons in visible light have a medium amount of energy. When photons have a little bit more energy, they become ultraviolet radiation, which you cannot see but which can give you a sunburn. With more energy than that, photons become X-rays, which travel right through you. If photons possess even more energy, they become gamma rays, which come out of stars when they explode.

But when photons have a little less energy than visible-light photons, they are known as infrared radiation. You can feel them as heat. Finally, we call the photons with the least energy “radio waves.” Radio waves come from strange spots in space – the coldest and oldest places and the stars with the most material stuffed into a small space. Radio waves tell us about parts of the universe we would not even know existed if we only used our eyes or telescopes that see visible photons.

Wavelength and Frequency

Radio astronomers use these radio photons to learn about the invisible universe. Photons travel in waves, like they are riding a roller coaster that just uses the same two pieces of track over and over [ 1 ]. The size of a photon’s wave – its wavelength – tells you about its energy. Figure 1 shows waves with two different wavelengths. If the wave is long, it does not have much energy; if it is short, it has a lot of energy. Radio waves do not have much energy, and that means they travel in big waves with long wavelengths. Radio waves can be hundreds of feet across or just a few centimeters across.

• Figure 1 - Photons travel in waves. The length of each wave is called a wavelength.

Astronomers also talk about how many of these waves pass a spot every second – the radio wave’s “ frequency .” You can think of frequency by imagining a pond of water. If you throw a rock into the water, ripples travel across the pond. If you stand in the water, the waves hit your ankles. The number of waves that smack into you in one second tells you the frequency of the waves. One wave per second is called 1 Hertz . A million waves per second is 1 MHz. If the waves are long, fewer of them hit you every second, so long waves have smaller frequencies. Radio waves have long wavelengths and small frequencies.

Radio Pioneers

The first radio astronomer did not mean to be the first radio astronomer. In 1933, a man named Karl Jansky was working on a project for Bell Laboratories, a lab in New Jersey named after Alexander Graham Bell, who invented the telephone. Engineers there were developing the first phone system that worked across the Atlantic Ocean. When people first tried making phone calls on that system, they heard a hissing sound in the background at certain times of the day. Bell Labs thought that noise was bad for business, so they sent Karl Jansky to find out what was causing it. He soon noticed that the hiss began when the middle of our galaxy rose in the sky and ended when it set (everything in the sky rises and sets just like the Sun and Moon do). He figured out that radio waves coming from the center of the galaxy were messing up the phone connection and causing the hiss. He – and the phone – had detected radio waves from space [ 1 ]. Jansky opened up a new, invisible universe. You can see a picture of the antenna used by Karl Jansky to detect radio waves from space in Figure 2 .

• Figure 2 - The founder of radio astronomy, Karl Jansky, stands with the antenna he built that detected the first radio waves identified as coming from space. Source: NRAO.

Inspired by Janksy’s research, a man named Grote Reber built a radio telescope in his backyard in Illinois. He finished the telescope, which was 31 ft across, in 1937 and used it to look at the whole sky and see where radio waves came from. Then, from the data he collected from his radio telescope, he made the first map of the “radio sky” [ 2 ].

Radio Telescope Talk

You can see visible light because the visible-light photons travel in small waves, and your eye is small. But because radio waves are big, your eye would need to be big to detect them. So while regular telescopes are a few inches or feet across, radio telescopes are much larger. The Green Bank Telescope in West Virginia is more than 300 ft wide and can be seen in Figure 3 . The Arecibo Telescope in the jungle in Puerto Rico is almost 1,000 ft across. They look like gigantic versions of satellite TV dishes, but they work like regular telescopes.

• Figure 3 - While instruments like the Green Bank Telescope, pictured here, may not look like traditional telescopes, they work much the same way but detect radio waves instead of visible light. They then turn those radio waves, which human eyes cannot see, into pictures and graphs that scientists can interpret. Source: NRAO.

To use a regular telescope, you point it at an object in space. Light from that object then hits a mirror or lens, which bounces that light to another mirror or lens, which then bounces the light again and sends it to your eye or a camera.

When an astronomer points a radio telescope at something in space, radio waves from space hit the telescope’s surface. The surface – which may be metal with holes in it, called mesh, or solid metal, like aluminum – acts like a mirror for radio waves. It bounces them up to a second “radio mirror,” which then bounces them into what astronomers call a “ receiver .” The receiver does what a camera does: it turns the radio waves into a picture. This picture shows how strong the radio waves are and where they are coming from in the sky.

Radio Vision

When astronomers look for radio waves, they see different objects and events than they see when they look for visible light. Places that seem dark to our eyes, or to regular telescopes, burn bright in radio waves. Places where stars form, for example, are full of dust. That dust blocks the light from getting to us, so the whole area looks like a black blob. But when astronomer turns radio telescopes to that spot, they can see straight through the dust: they can see a star being born.

Stars are born in giant clouds of gas in space. First, that gas clumps together. Then, because of gravity, more and more gas is attracted to the clump. The clump grows bigger and bigger and hotter and hotter. When it is huge and hot enough, it starts smashing hydrogen atoms, the smallest atoms that exist, together. When hydrogen atoms crash into each other, they make helium, a slightly bigger atom. Then, this clump of gas becomes an official star. Radio telescopes take pictures of these baby stars [ 3 ].

Radio telescopes show the secrets of the nearest star, too. The light we see from the Sun comes from near the surface, which is about 9,000oF. But above the surface, the temperature reaches 100,000oF. Radio telescopes help us learn more about these hot parts, which send out radio waves.

The planets in our solar system also have radio personalities. Radio telescopes show us the gases that swirl around Uranus and Neptune and how they move around. Jupiter’s north and south poles light up in radio waves. If we send radio waves toward Mercury, and then catch the radio waves that bounce back using a radio telescope, we can make a map almost as good as Google Earth [ 4 ].

When they look much farther away, radio telescopes show us some of the weirdest objects in the universe. Most galaxies have supermassive black holes in their centers. Black holes are objects that have a lot of mass squished into a tiny space. This mass gives them so much gravity that nothing, not even light, can escape their pull. These black holes swallow stars, gas, and anything else that comes too close. When that unlucky stuff feels the black hole’s gravity, it first spirals around the black hole. As it gets closer, it goes faster and faster. Huge jets, or columns, of electromagnetic radiation and matter that does not make it in to the black hole (sometimes taller than a whole galaxy is wide) form above and below the black hole. Radio telescopes show those jets in action ( Figure 4 ).

• Figure 4 - Galaxies that have supermassive black holes at their centers can shoot out jets of material and radiation, like those seen here, that are taller than the galaxy is wide. Source: NRAO.

Massive objects like these black holes warp the fabric of space, called space-time. Imagine setting a bowling ball, which weighs a lot, on a trampoline. The trampoline sags down. Weighty stuff in space makes space-time sag just like the trampoline. When radio waves coming from distant galaxies travel over that sag to get to Earth, the shape acts just like the shape of a magnifying glass on Earth: telescopes then see a bigger, brighter picture of the distant galaxy.

Radio telescopes also help solve one of the biggest mysteries in the universe: What is dark energy ? The universe is getting larger every second. And it gets larger faster and faster every second because “dark energy” is the opposite of gravity: Instead of pulling everything together, it pushes everything farther apart. But how strong is dark energy? Radio telescopes can help scientists to answer this question by looking at “ megamasers ” that occur naturally in some parts of space, a megamaser is kind of like a laser on Earth, but it sends out radio waves instead of the red or green light that we can see. Scientists can use megamasers to pin down the details of dark energy [ 5 ]. If scientists can figure out how far away those megamasers are, they can tell how far away different galaxies are, and then they can figure out how fast those galaxies are speeding away from us.

A Full Toolbox

If we only had telescopes that picked up visible light, we would be missing out on much of the action in the universe. Imagine if doctors had only a stethoscope as a tool. They could learn a lot about the patient’s heartbeat. But they could learn so much more if they also had an X-ray machine, a sonogram, an MRI instrument, and a CT scanner. With those tools, they could get a more complete picture of what was happening inside the patient’s body. Astronomers use radio telescopes together with ultraviolet, infrared, optical, X-ray, and gamma-ray telescopes for the same reason: to get a complete picture of what is happening in the universe.

Electromagnetic spectrum : ↑ The visible light that we can see is just a tiny part of the “electromagnetic spectrum.” Visible light is made of photons with medium energy. Photons with more energy are ultraviolet radiation, X-rays, and gamma rays (gamma rays have the most energy). Photons with less energy are infrared and radio waves (radio waves have the least energy).

Photon : ↑ Light is made of particles called photons, which travel in waves.

Wavelength : ↑ The size of the wave a photon travels in.

Frequency : ↑ The number of light waves that pass by a spot in one second.

Hertz : ↑ 1 Hz means that one wave passes by a spot in one second. One megahertz means one million waves pass by every second.

Receiver : ↑ The part of a radio telescope that takes the radio waves and turns them into a picture.

Dark energy : ↑ Dark energy acts like the opposite of gravity and pushes everything in the universe farther apart.

Megamaser : ↑ A natural laser in space that sends out radio waves, instead of red or green light like the kind that comes from a laser pointer.

[1] ↑ Jansky, K. G. 1993. Radio waves from outside the solar system. Nature 32, 66. doi: 10.1038/132066a0

[2] ↑ Reber, G. 1944. Cosmic static. Astrophys. J. 100, 297. doi: 10.1086/144668

[3] ↑ McKee, C. F., and Ostriker, E. 2007. Theory of star formation. Annu. Rev. Astron. Astrophys. 45, 565–687. doi: 10.1146/annurev.astro.45.051806.110602

[4] ↑ Ostro, S. J. 1993. Planetary radar astronomy. Rev. Mod. Phys. 65, 1235–79. doi: 10.1103/RevModPhys.65.1235

[5] ↑ Henkel, C., Braatz, J. A., Reid, M. J., Condon, J. J., Lo, K. Y., Impellizzeri, C. M. V., et al. 2012. Cosmology and the Hubble constant: on the megamaser cosmology project (MCP). IAU Symp. 287, 301. doi: 10.1017/S1743921312007223