Electromagnetic Waves

Type of physical science: Classical physics

Field of study: Electromagnetism

Combinations of fluxuating electric and magnetic fields created by accelerated electric charges propagate out from those charges at the speed of light in the form of waves--electromagnetic waves or radiation. Earth's natural and human-influenced environment is bathed in all types of electromagnetic radiation: power waves, radio waves, microwaves, infrared, visible, ultraviolet, X rays, and γ rays.

Overview

Many natural phenomena exhibit wavelike behavior. Water waves, earthquake waves, and sound waves all require a medium or substance through which to propagate. These are examples of mechanical waves. Light can also be described as waves--waves of changing electric and magnetic fields that propagate outward from their sources. These electromagnetic waves, however, do not require a medium. They propagate at 300 million meters per second through the vacuum of space. Electromagnetic waves are transverse waves. In other words, the changing electric and magnetic fields oscillate perpendicular to each other and to the direction of the propagating wave.

The ultimate source of all electromagnetic waves is accelerated electric charges. An accelerated charge is one that is increasing or decreasing its speed or changing its direction of motion or both. Imagine two charges at rest in the vicinity of each other. They are immersed in each other's electric force field. If one charge suddenly begins to oscillate (vibrate) up and down, the second charge experiences the change in the field of the first charge after some very small but finite time elapses. The oscillating charge was accelerated. The moving charges' electric fields changed, as did their magnetic fields. These changing electric and magnetic fields generate each other through Faraday's law of induction and Ampere's law. These changing fields dissociate from the oscillating charge and propagate out into space at the speed of light.

All periodic waves, whether they are electromagnetic or mechanical, are characterized by such properties as wavelength, frequency, and speed. For electromagnetic waves, wavelength measures the distance between successive pulses of electric or magnetic fields. A wave's frequency represents how many wave pulses pass by a given point each second and is measured in cycles per second or waves per second. One wave per second is called one hertz. (One can also view frequency as the rate of oscillation of the accelerated charge that created the electromagnetic wave.) Electromagnetic waves all travel at 300 million meters per second, which is the speed of light in a vacuum. They travel more slowly when they pass through various media such as air, glass, and water. A relationship among frequency, wavelength, and speed exists for electromagnetic waves: the product of the wavelength and frequency is equal to the speed of light. Thus, wavelength and frequency are inversely related. The longer the wavelength, the lower the frequency and vice versa.

An entire spectrum of electromagnetic waves exists which ranges from very long-wavelength (low-frequency) waves to very short-wavelength (high-frequency) waves. The figure labels the various regions of the electromagnetic spectrum and indicates the approximate wavelength and frequency ranges in meters and hertz, respectively.

The names given to the different regions are those in common usage. No sharp wavelength or frequency boundaries exist between these arbitrary divisions. Note that all electromagnetic waves are referred to collectively as light or electromagnetic radiation, not merely the narrow range of wavelengths and frequencies indentified as visible light.

The wave nature of light describes many aspects of its behavior. Nevertheless, radiation also has particle-like characteristics. Rather than infinite or nearly infinite series of electromagnetic waves emanating from some accelerated charge, light appears to come in particle-like bursts of energy. These individual bursts, or quanta, of energy are called photons.

Each photon possesses an amount of energy that directly depends on the frequency of its associated electromagnetic wave. Doubling the frequency of the photon of radiation doubles its energy. Thus, of all types of electromagnetic waves, photons of power waves possess the least energy and γ-ray photons possess the greatest energy.

Since all life on Earth is bathed constantly in all forms of electromagnetic radiation, scientists must be cognizant of the potential risks, as well as the benefits, of exposures to electromagnetic waves.

Applications

Almost every aspect of the daily existence of humans is affected, if not guided, by forms of electromagnetic waves or radiation. The sun is an immense ball of gaseous elements, primarily hydrogen and helium, whose atomic parts are charges that are undergoing accelerations. The electromagnetic waves thus generated span the entire spectrum. They are the powerhouse of the physical and life processes on Earth.

Alternating current transmission lines create power waves, which are the very long wavelength end of the electromagnetic spectrum. The frequency of power waves is the frequency of the alternating current produced by the alternating current generators in electric power plants around the world. In the United States and some other countries, the frequency is 60 hertz. Other countries, especially those in Europe, generate a frequency of 50 hertz. Frequency can be changed by varying the rotational speed of the generator at the power station. Frequencies of up to 1,000 hertz (1 kilohertz) can be generated this way. Humans are exposed to these waves from a variety of sources: electric appliances and blankets, wiring in homes and workplaces, transmission lines, and local distribution and transformer stations. Research has yet to determine conclusively what effect the power waves thus created have on living systems. Some studies show a correlation to higher rates of cancers and other infirmities. Other investigations suggest little or no effects.

Electromagnetic waves with frequencies between 1 kilohertz and 1 billion hertz are generated commonly by series circuits of coils (inductors) and capacitors connected to antennas.

Electrons oscillate in a given circuit and on the antenna wire, thereby generating radio waves.

Radio waves include the familiar amplitude modulation (AM), frequency modulation (FM), short wave, and citizen's band, as well as the many television bands. Another source of radio waves is vibrating quartz crystals used in clocks and wristwatches. They typically produce 1-kilohertz to 10-million-hertz -megahertz) radio waves.

At still higher frequencies and shorter wavelengths, microwaves are often generated in electron tubes called magnetrons. Microwaves from 1 meter to 10 centimeters, known as radar, are used for vital information transfer and communication in civilian and military life.

Microwave radiation from 10 centimeters to 1 millimeter accelerates electrons in molecules and thus has uses other than communication. Water absorbs microwave energy of about 10 centimeters easily and converts it into thermal energy in the molecules of moisture-containing substances. Microwave ovens cook food in this manner. They confine this electromagnetic energy in the closed oven cavity to keep the radiation from damaging the living tissue of the operator.

Electromagnetic waves with lengths ranging from about 1 millimeter down to 1 ten-millionth of a meter fall within the infrared region of the spectrum. Infrared is known for the heat it deposits when it encounters matter. Infrared-sensitive film, siting devices, and heat monitors are used routinely by various agencies such as the weather service, the U.S. Geological Survey, and the Departments of Energy, Defense, and Agriculture for data gathering, surveillance, and energy conservation. All bodies, living and inert, exist at some temperature and radiate infrared over a range of frequencies. Nevertheless, each radiates at a certain characteristic maximum wavelength. Earth radiates infrared, but water and carbon dioxide in the atmosphere absorb much of it, heating up the atmosphere. This phenomenon is known as the greenhouse effect.

While the atmosphere is opaque to Earth's infrared radiation, it acts as a window to other infrared frequencies and to some radio waves as well. Astronomers receive and transmit electromagnetic waves through these windows to study the universe. The IRAS (Infrared Astronomical Satellite) of the early 1980's studied the universe in infrared. In 1989, the COBE satellite confirmed cosmological theories that predict that the universe is suffused by infrared and microwave radiation at nearly 3 degrees above absolute zero temperature (3 Kelvins). This electromagnetic "glow" is the redshifted remnant of the cataclysm known as the big bang, which occurred some 15 to 20 billion years ago and is believed to be the origin of the universe.

The atmosphere has a window to visible light as well. Visible light is a narrow margin in the electromagnetic spectrum ranging from 7 ten-millionths of a meter to 4 ten-millionths of a meter in length. It is the only electromagnetic radiation to which the visual receptors in human eyes are sensitive. Presumably, because the sun gives off the bulk of its radiation in the visible region, Earth has evolved in response to this environment. Visible light waves contain all the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet. Their unequal combination from the sun produces white light with a preponderance of green and yellow; incandescent bulbs produce white light with more reds added. Fluorescent sources produce "bluer" white light. Visible light can be both the cause and the result of electron energy transitions because of accelerations with atoms. The study of these processes is called spectral analysis, and it is a vital tool to astronomers, physicists, chemists, and metallurgists. Ultraviolet waves may result from accelerated electrons within atoms (as may infrared), and spectral studies in ultraviolet are as useful as those in the visible and infrared regions. These short waves extend from 4 ten-millionths of a meter down to 1 hundred-millionth of a meter.

Small amounts of ultraviolet radiation from the sun and other sources in the universe penetrate to the surface of Earth. Ultraviolet light can cause photochemical reactions, such as in film exposure, the production of ozone in the upper atmosphere, and the tanning and burning of skin by the production of additional pigmentation called melanin. Sunscreens contain chemicals whose molecules absorb the ultraviolet radiation before it reaches the skin. Ultraviolet radiation, produced by excited mercury gas electrons in every fluorescent tube, excites the atoms of the white powder on the inside of the glass to radiate visible white light. Ordinary glass absorbs ultraviolet radiation. Hence, fluorescent lights do not emit it, but the white light comes through.

Because the atmosphere is largely opaque to ultraviolet radiation, astronomers must go above it to study the ultraviolet universe. Since its launch in the late 1970's, the International Ultraviolet Explorer has added much to knowledge of the far ultraviolet in space. The Hubble Space Telescope, when fully operational, should extend knowledge of the processes of the universe from the infrared through the visible and into the far ultraviolet.

X-ray wavelengths are even shorter than ultraviolet wavelengths. X rays are often produced by sudden large accelerations (decelerations, deflections) of high-speed electrons as they encounter matter. Large voltages are required to energize the electrons initially before they are caused to crash into matter. Cathode-ray tubes such as in televisions, oscilloscopes, and medical X-ray machines accomplish this task using high-accelerating voltages ,000 volts or more). The high-frequency X rays penetrate low-density organic material but are absorbed or reflected from more dense substances such as bone or metal. They expose film to produce the familiar dental and medical X rays. An increasingly common X-ray technology is the formation of three-dimensional X-ray images called CAT (computerized axial tomography) scans.

Like all other electromagnetic waves, X rays exhibit the optical properties of reflection, refraction, diffraction, and interference. Utilizing these properties, chemists, biologists, and mineralogists bombard various crystals and other complex molecules with X rays to determine atomic and molecular structure.

The division between the γ-ray and the X-ray regions of the spectrum is a fuzzy one. Gamma rays are produced by acceleration of charges within the nuclei of atoms. They are emitted by radioactive, unstable nuclei, and nuclear reactions. Many impinge on Earth in the form of cosmic rays. They are extremely high-energy bursts of radiation, deleterious to living tissue.

Context

The phenomena associated with electricity and magnetism were studied over much of the nineteenth century. Knowledge that the two fields of study were connected began with the serendipitous discovery by Hans Christian Orsted in the early 1820's. He learned that magnetism is ultimately caused by moving electric charges or currents when he observed a compass needle react to a current-carrying wire. The simultaneous though separate discoveries of Michael Faraday and Joseph Henry concerning electromagnetic induction in the 1830's led to the theory of James Clerk Maxwell, which united electricity, magnetism, and optics into one grand theory of light, or radiant energy: the explanation of light as electromagnetic waves.

Maxwell published TREATISE ON ELECTRICITY AND MAGNETISM (1873), in which he showed that four fundamental mathematical equations described all the known electric and magnetic phenomena. The first equation is Gauss's law for electricity, which states that positive and negative charges create electric fields; Gauss's law for magnetism states that currents create magnetic fields, which have associated north and south poles, but single poles (monopoles) do not exist; Ampere's law states that time-varying magnetic fields induce time-varying electric fields; and Faraday's law of induction states that time-varying electric fields induce time-varying magnetic fields. Additionally, Maxwell's equations predicted the existence of combined, changing electric and magnetic fields in the form of waves that traveled at the speed of light-- electromagnetic waves. He speculated that accelerated charges ultimately create these electromagnetic waves, that they should exist over a wide range of frequencies and wavelengths, that they traveled at the speed of light in a vacuum, and that they exhibited all the optical properties of visible light, such as reflection, refraction, and diffraction.

Thus, electricity, magnetism, and optics represented different manifestations of a more generalized electromagnetic field. Maxwell's synthesis of electromagnetism and optics represents the crowning achievement of nineteenth century physics. Maxwell's theory was verified experimentally ten years after his death by Heinrich Rudolf Hertz in 1887. Hertz built an induction coil device, which was essentially a step-up transformer whose high output voltage caused sparks to jump back and forth (oscillate) across an air gap between two metal plates. One wire, bent so that it too had an air gap between its ends, was placed near another wire. Hertz noticed sparks jumping across the ends of this wire at the same frequency as the induction coil's sparks. He concluded that electromagnetic waves propagated through the air from the coil to the bent wire. These waves proved to be radio waves of about 1 meter in wavelength. He demonstrated that these waves exhibited all the usual properties of light; namely, they reflected, focused on parabolic mirrors, and refracted through glass. He caused them to interfere, setting up a standing wave pattern that enabled him to calculate their speed to be the speed of light. Later experiments demonstrated that a wide range of electromagnetic wavelengths (and frequencies) exist and led to the technologies of radio, television, radar, and myriad other technologies important to society.

As monumental as Maxwell's theory of electromagnetic waves was, it became clear by the end of the nineteenth century that Maxwell's wave description of light was not the sole explanation of its properties. Understanding the photoelectric effect and the nature of black body radiation ultimately demanded a particle explanation of light's behavior, wherein "particles" of light, called photons, have quantized energies proportional to the frequency of the associated wave. Photons of power waves have the least energy; γ-ray photons have the greatest energy.

Principal terms

ACCELERATED CHARGE: an electric charge that is increasing or decreasing its speed and/or changing its direction of travel

ELECTROMAGNETIC SPECTRUM: the range of types of electromagnetic waves from extremely long wavelengths and low frequencies to extremely short wavelengths and high frequencies

ELECTROMAGNETIC WAVE: a combination of fluxuating electric and magnetic fields, each generating the other and propagating at the speed of light

FREQUENCY: the number of periodic wave pulses passing by in a unit of time; the usual unit of frequency is the hertz; one hertz is one wave pulse per second

LIGHT: the generic term for any type of electromagnetic wave or radiation; common usage refers to the portion of the electromagnetic spectrum called visible light

RADIATION: a term that can refer to any type of electromagnetic wave or light

WAVELENGTH: the distance between any two corresponding points on successive portions of a periodic wave, such as from one crest to the next crest; the unit of wavelength is the meter

Bibliography

Asimov, Isaac. ASIMOV ON PHYSICS. Garden City, N.Y.: Doubleday, 1976. The prolific Asimov produced this delightful exposition on understanding physics as part of a collection of essays on understanding many areas of science. Worthwhile for the reader to review are chapter 6, "The Rigid Vacuum," and chapter 8, "The Light Fantastic."

Asimov, Isaac. UNDERSTANDING PHYSICS. Vol. 2, LIGHT, MAGNETISM, AND ELECTRICITY. Vol. 3, THE ELECTRON, PROTON, AND NEU. NEW YORK: New American Library, 1969. a classic. These volumes contain clear, well-written discussions of electromagnetic radiation and the people, places, events, and experiments involved.

Gamow, George. THE GREAT PHYSICISTS FROM GALILEO TO EINSTEIN. New York: Dover, 1988. Gamow is a great interpreter of science concepts into understandable language. Succeeds in communicating the magic of physics through the lives and times of the discoverers.

Mulligan, Joseph F. "Heinrich Hertz and the Development of Physics." PHYSICS TODAY 42 (March, 1989): 50-57. Provides details on the experiments conducted by Hertz on electromagnetic waves, as well as his contributions to theoretical electromagnetism and his discovery of the photoelectric effect. Mulligan establishes how Maxwell's theory and Hertz's experimental work helped lay the foundation for quantum theory and relativity.

Olenick, Richard P., Tom M. Apostol, and David L. Goodstein. BEYOND THE MECHANICAL UNIVERSE. New York: Cambridge University Press, 1986. Though a calculus-based physics course, the text emphasizes the phenomena and the physics principles in their historical and philosophical contexts.

Electromagnetic spectrum

Parts of a wave

Colors in the electromagnetic spectrum

Generating and Detecting Electromagnetic Waves

Motors and Generators

Radiation: Interactions with Matter

Radio and Television