electromagnetic radiation
, in classical
physics
, the flow of
energy
at the universal
speed of light
through free space or through a material medium in the form of the
electric
and
magnetic fields
that make up electromagnetic waves such as
radio waves
,
visible light
, and
gamma rays
. In such a
wave
, time-varying electric and magnetic fields are mutually linked with each other at right angles and perpendicular to the direction of motion. An electromagnetic wave is
characterized
by its intensity and the
frequency
ν of the time variation of the electric and magnetic fields.
In terms of the modern
quantum theory
, electromagnetic
radiation
is the flow of
photons
(also called light quanta) through space. Photons are packets of energy
h
ν that always move with the universal speed of light. The symbol
h
is
Planck’s constant
, while the value of ν is the same as that of the frequency of the electromagnetic wave of classical theory. Photons having the same energy
h
ν are all alike, and their number
density
corresponds to the intensity of the radiation. Electromagnetic radiation exhibits a multitude of phenomena as it interacts with charged particles in
atoms
,
molecules
, and larger objects of matter. These phenomena as well as the ways in which electromagnetic radiation is created and observed, the manner in which such radiation occurs in nature, and its technological uses depend on its frequency ν. The
spectrum
of frequencies of electromagnetic radiation extends from very low
values
over the range of radio waves,
television
waves, and
microwaves
to visible light and beyond to the substantially higher values of
ultraviolet light
,
X-rays
, and gamma rays.
The basic properties and behaviour of electromagnetic radiation are discussed in this article, as are its various forms, including their sources, distinguishing characteristics, and practical applications. The article also traces the development of both the classical and
quantum
theories of radiation.
General considerations
Occurrence and importance
Close to 0.01 percent of the mass/energy of the entire
universe
occurs in the form of electromagnetic radiation. All human
life
is immersed in it, and modern communications technology and medical services are particularly dependent on one or another of its forms. In fact, all living things on Earth depend on the electromagnetic radiation received from the
Sun
and on the transformation of
solar energy
by
photosynthesis
into
plant
life or by biosynthesis into
zooplankton
, the basic step in the
food chain
in
oceans
. The
eyes
of many animals, including those of humans, are adapted to be sensitive to and hence to see the most abundant part of the Sun’s electromagnetic radiation?namely,
light
, which
comprises
the visible portion of its wide range of frequencies. Green plants also have high sensitivity to the maximum intensity of solar electromagnetic radiation, which is absorbed by a substance called
chlorophyll
that is essential for plant growth via photosynthesis.
Britannica Quiz
Matter and More Quiz
Practically all the fuels that modern society uses?
gas
,
oil
, and
coal
?are stored forms of energy received from the Sun as electromagnetic radiation millions of years ago. Only the energy from
nuclear reactors
does not originate from the Sun.
Everyday life is pervaded by artificially made electromagnetic radiation: food is heated in
microwave ovens
,
airplanes
are guided by
radar
waves,
television
sets receive electromagnetic waves
transmitted
by broadcasting stations, and
infrared
waves from heaters provide warmth. Infrared waves also are given off and received by automatic self-focusing
cameras
that electronically measure and set the correct distance to the object to be photographed. As soon as the Sun sets,
incandescent
or
fluorescent lights
are turned on to provide artificial illumination, and cities glow brightly with the colourful fluorescent and
neon
lamps of advertisement signs. Familiar too is
ultraviolet radiation
, which the eyes cannot see but whose effect is felt as pain from
sunburn
. Ultraviolet light represents a kind of electromagnetic radiation that can be harmful to
life
. Such is also true of
X-rays
, which are important in
medicine
as they allow physicians to observe the inner parts of the body but exposure to which should be kept to a minimum. Less familiar are
gamma rays
, which come from
nuclear reactions
and
radioactive
decay
and are part of the harmful high-energy radiation of radioactive materials and
nuclear weapons
.
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