Electricity · Magnetism
Electromagnetic radiation (sometimes abbreviated EMR
and often simply called light) is a ubiquitous phenomen-
on that takes the form of self-propagating waves in a va-
cuum or in matter. It consists of electric and magnetic
field components which oscillate in phase perpendicular
to each other and perpendicular to the direction of en-
ergy propagation. Electromagnetic radiation is classified
into several types according to the frequency of its
wave; these types include (in order of increasing fre-
quency and decreasing wavelength): radio waves, mi-
crowaves, terahertz radiation, infrared radiation, visible
light, ultraviolet radiation, X-rays and gamma rays. A
small and somewhat variable window of frequencies is
sensed by the eyes of various organisms; this is what we
call the visible spectrum, or light.
EM radiation carries energy and momentum that
may be imparted to matter with which it interacts.
Electromagnetic waves were first postulated by James
Clerk Maxwell and subsequently confirmed by Heinrich
Hertz. Maxwell derived a wave form of the electric and
magnetic equations, revealing the wave-like nature of
electric and magnetic fields, and their symmetry. Be-
cause the speed of EM waves predicted by the wave
equation coincided with the measured speed of light,
Maxwell concluded that light itself is an EM wave.
According to Maxwell’s equations, a time-varying
electric field generates a magnetic field and vice versa.
Therefore, as an oscillating electric field generates an
oscillating magnetic field, the magnetic field in turn
generates an oscillating electric field, and so on. These
oscillating fields together form an electromagnetic
A quantum theory of the interaction between elec-
tromagnetic radiation and matter such as electrons is
described by the theory of quantum electrodynamics.
Shows three electromagnetic modes (blue, green and red) with
a distance scale in micrometres along the x-axis.