Electromagnetic radiation |
Electromagnetic radiation is a combination of oscillating
electric and magnetic fields propagating perpendicular to each
other through space and carrying energy from one place to another. Light is a form of electromagnetic radiation.
The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism.
Electromagnetic waves were predicted by Maxwell's
equations and subsequently discovered by Heinrich Hertz.
Any electric charge which accelerates radiates electromagnetic radiation. Electromagnetic information about the charge travels
at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced
time, which is unphysical in light of causality), which adds to the expressions
for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic
radiation. When any wire (or other conducting object such as an antenna) conducts alternating
current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the
circumstances, it may behave as waves or as particles. As a wave, it is characterized by a velocity (the velocity of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 ×
10-34 J·s is Planck's constant, and ν
is the frequency of the wave.
Generally, electromagnetic radiation is classified by wavelength into radio, microwave, infrared light, visible light, ultraviolet light, X-rays and gamma rays. The details of this
classification are contained in the article on the electromagnetic spectrum.
The effect of radiation depends on the amount of energy per quantum it carries. High energies correspond to high frequencies
and short wavelengths, and vice versa. One rule is always obeyed, regardless of the circumstances. Radiation in vacuum always
travels at the speed of light, relative to the observer,
regardless of the observer's velocity. (This observation led to Albert
Einstein's development of the theory of special
relativity).
Much information about the physical properties of an object can be obtained from its electromagnetic spectrum; this can be
either the spectrum of light emitted from, or transmitted through the object. This involves spectroscopy and is widely used in astrophysics. For
example; many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.
When electromagnetic radiation impinges upon a conductor it induces
an electric current flow on the surface of that conductor. This effect (the skin
effect) is used in antennas. Electromagnetic radiation may also cause certain molecules to absorb energy and thus to heat up;
this is exploited in microwave ovens.
See Also
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