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Electromagnetic Radiation
I
INTRODUCTION
Electromagnetic Radiation, 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. 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
Electromagnetic waves need no material medium for their
transmission. Thus, light and radio waves can travel through interplanetary and
interstellar space from the Sun and stars to the Earth. Regardless of their
frequency and wavelength, electromagnetic waves travel at the same speed in a
vacuum. The value of the metre has been defined so that the speed of light is
exactly 299,792.458 km (approximately 186,282 mi) per second in a vacuum. All
the components of the electromagnetic spectrum also show the typical properties
of wave motion, including diffraction and interference. The wavelengths range
from billionths of a centimetre to many kilometres. The wavelength and frequency
of electromagnetic waves are important in determining their heating effect,
visibility, penetration, and other characteristics.
III
THEORY
The British physicist James Clerk Maxwell laid out the
theory of electromagnetism in a series of papers published in the 1860s. He
deduced that electromagnetic waves must exist and stated that visible light
consisted of such waves.
Physicists had known since the early 19th century that
light travels 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 thought that there was an extremely diffuse substance,
called ether, which was the unobserved medium. Maxwell's theory made such an
assumption unnecessary, but the ether concept was not abandoned immediately,
because it fitted 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 undermined 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 the same, regardless of the velocity of
the source or the observer.
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
1905, Albert Einstein was able to explain some puzzling experimental results
concerning the photoelectric effect by suggesting that electromagnetic radiation
can behave like a particle.
Other phenomena that 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
behave sometimes like a particle and sometimes like a wave. The parallel
concept—that matter also exhibits particle-like and wave-like
characteristics—was developed in 1925 by the French physicist Louis de
Broglie. SeeWave-Particle Duality.