Electromagnetic Waves

Electromagnetic waves are related patterns of electric and magnetic force. They are generated by the oscillation (movement back and forth) of electric charges. Electromagnetic waves travel through space at the speed of light--186,282 miles (299,792 kilometers) per second. The simplest electromagnetic waves are plane waves. They move through space in straight lines. The strength of the wave varies in space and time with alternating crests and troughs. The distance from crest to crest is called the wavelength. See Light.

The electromagnetic spectrum consists of bands of different wavelengths. The chief kinds of electromagnetic waves are--in order of increasing wavelength--gamma rays, X rays, ultraviolet light, visible light, infrared rays, microwaves, and radio waves. Gamma rays are less than 10-trillionths of a meter in length, and some long radio waves measure more than 10,000 kilometers.

All types of electromagnetic waves have the properties of visible light. They can be reflected, diffracted (spread), and refracted (bent). The direction of magnetic force in all electromagnetic waves is perpendicular to the direction in which the wave is moving. The direction of electric force is perpendicular to both the direction of magnetic force and the direction of wave motion. The strength of magnetic force always equals the strength of electric force.

Uses of electromagnetic waves. Doctors use gamma rays, which are given off by radium, to treat cancer. They also use X rays to treat cancer and to help locate and diagnose internal disorders. Ultraviolet rays are used in sun lamps and fluorescent lights and as a disinfectant. Infrared rays, which are given off by hot objects, are used in infrared lamps to treat skin diseases and to bake enamel. Microwaves are used to cook food. Radio waves are used in radio and TV broadcasting.

The technological use of electromagnetic waves depends on the ease with which the various wavelengths can be detected and produced. Wavelength is related to the vibration rate of electrons in the energy source. The slower the vibration, the longer the wavelength. Long waves are the easiest to produce, and the use of radio waves in communications began soon after 1900. Shorter waves were not effectively utilized until the development of such devices as the klystron, a type of microwave tube (see Vacuum Tube). Development of the laser during the 1960's provided new uses for short waves. For example, the laser enables ultraviolet and infrared rays to transmit voice messages and television signals (see Laser).

History. In 1864, the British physicist James Clerk Maxwell predicted the existence of electromagnetic waves. Maxwell assumed that electric and magnetic fields act together to produce radiant energy in the form of electromagnetic waves. He defined visible light as a type of electromagnetic wave and predicted the existence of invisible waves. In the late 1880's, the German physicist Heinrich R. Hertz verified Maxwell's prediction. Hertz demonstrated that oscillation of an electric charge produces electromagnetic waves that are much longer than visible light waves. His discovery led to the development of radio and television.

Contributor: Gerald Feinberg, Ph.D., Former Prof. of Physics, Columbia Univ.

Additional Resources

Branley, Franklyn M. The Electromagnetic Spectrum: Key to the Universe. Crowell, 1979.

Carter, R. G. Electromagnetic Waves: Microwave Components and Devices. Chapman & Hall, 1990.

Tolstoy, Ivan. James Clerk Maxwell: A Biography. Univ. of Chicago Pr., 1981.

Electromagnetic Waves

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