Electromagnetic Radiation
| I | INTRODUCTION |
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
1923 by the French physicist Louis Victor, Prince de Broglie.
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