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Electromagnetic
Radiation, energy waves produced by the oscillation or acceleration of
an electric charge. Electromagnetic waves have both electric and magnetic
components. Electromagnetic radiation can be arranged in a spectrum that
extends from waves of extremely high frequency
and short wavelength to extremely low frequency and long wavelength (see
Wave
Motion). Visible light is only a small part of the electromagnetic
spectrum. In order of decreasing frequency, the electromagnetic spectrum
consists of gamma rays, hard and soft X
rays, ultraviolet
radiation, visible light, infrared
radiation, microwaves,
and radio waves.
 II.
Properties
There
are three phenomena through which energy can be transmitted:
electromagnetic radiation, conduction, and convection (see Heat
Transfer). Unlike conduction and convection, electromagnetic waves
need no material medium for transmission. Thus, light and radio waves can
travel through interplanetary and interstellar space from the sun and
stars to the earth. Regardless of the frequency, wavelength, or method of
propagation, electromagnetic waves travel at a speed of 3 × 1010
cm (186,272 mi) per second in a vacuum. All the components of the
electromagnetic spectrum, regardless of frequency, also have in common the
typical properties of wave motion, including diffraction
and interference.
The wavelengths range from millionths of a centimeter to many kilometers.
The wavelength and frequency of electromagnetic waves are important in
determining heating effect, visibility, penetration, and other
characteristics of the electromagnetic radiation.
III.
Theory
British
physicist James
Clerk Maxwell laid out the theory of electromagnetic waves in a series
of papers published in the 1860s. He analyzed mathematically the theory of
electromagnetic fields
and predicted that visible light was an electromagnetic phenomenon.
Physicists
had known since the early 19th century that light is propagated as a
transverse wave (a wave in which the vibrations move in a direction
perpendicular to the direction of the advancing wave front). They assumed,
however, that the wave required some material medium for its transmission,
so they postulated an extremely diffuse substance, called ether,
as the unobservable medium. Maxwell's theory made such an assumption
unnecessary, but the ether concept was not abandoned immediately, because
it fit in with the Newtonian concept of an absolute space-time frame for
the universe. A famous experiment conducted by the American physicist Albert
Abraham Michelson and the American chemist Edward Williams Morley in
the late 19th century served to dispel the ether concept and was important
in the development of the theory of relativity.
This work led to the realization that the speed of electromagnetic
radiation in a vacuum is an invariant.
IV.
Quanta of Radiation
At
the beginning of the 20th century, however, physicists found that the wave
theory did not account for all the properties of radiation. In 1900 the
German physicist Max
Planck demonstrated that the emission and absorption of radiation
occur in finite units of energy, known as quanta. In 1904,
German-born American physicist Albert
Einstein was able to explain some puzzling experimental results on the
external photoelectric
effect by postulating that electromagnetic radiation can behave like a
particle (see Quantum
Theory).
Other
phenomena, which occur in the interaction between radiation and matter,
can also be explained only by the quantum theory. Thus, modern physicists
were forced to recognize that electromagnetic radiation can sometimes
behave like a particle, and sometimes behave like a wave. The parallel
concept—that matter
also exhibits the same duality of having particlelike and wavelike
characteristics—was developed in 1925 by the French physicist Louis
Victor, Prince de Broglie.
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