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Electromagnetic Radiation
Electromagnetic radiation
Electromagnetic radiation is a combination
of
oscillating
electric and
magnetic fields propagating through space and carrying
energy from one place to another.
Light is a form of electromagnetic radiation. The
theoretical study of electromagnetic radiation is called
electrodynamics, a subfield of
electromagnetism.
Any electric charge which accelerates radiates
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 = hv, where E is the
energy of the photon, h is
Planck's constant - 6.626 × 10-34 J·s - and v is
the frequency of the wave.
Einstein later updated this formula to Ephoton = hv.
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 passes through a
conductor it
induces an electric current flow in the conductor. This effect is used in
antennas. Electromagnetic radiation may also cause certain molecules to
oscillate and thus to heat up; this is exploited in
microwave ovens.
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