James Clerk Maxwell first formally-postulated Electromagnetic waves.
These were
subsequently confirmed by Heinrich Hertz. Maxwell derived a wave form of the electric and
magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields, and
their symmetry. Because 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 time-varying
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 a propagating electromagnetic wave.
A quantum theory of the interaction between electromagnetic radiation and matter such as
electrons is described by the theory of quantum electrodynamics.
Properties
Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of
electric and magnetic fields. This diagram shows a plane linearly polarized wave propagating
from right to left. The electric field is in a vertical plane and the magnetic field in a horizontal
plane.
The physics of electromagnetic radiation is electrodynamics. Electromagnetism is the physical
phenomenon associated with the theory of electrodynamics. Electric and magnetic fields obey
the properties of superposition. Thus, a field due to any particular particle or time-varying
electric or magnetic field contributes to the fields present in the same space due to other causes.
Further, as they are vector fields, all magnetic and electric field vectors add together according to
vector addition. For example, in optics two or more coherent lightwaves may interact and by
constructive or destructive interference yield a resultant irradiance deviating from the sum of the
component irradiances of the individual lightwaves.
Since light is an oscillation it is not affected by travelling through static electric or magnetic
fields in a linear medium such as a vacuum. However in nonlinear media, such as some crystals,
interactions can occur between light and static electric and magnetic fields — these interactions
include the Faraday effect and the Kerr effect.
In refraction, a wave crossing from one medium to another of different density alters its speed
and direction upon entering the new medium. The ratio of the refractive indices of the media
determines the degree of refraction, and is summarized by Snell's law. Light disperses into a
visible spectrum as light is shone through a prism because of the wavelength dependent
refractive index of the prism material (Dispersion).
EM radiation exhibits both wave properties and particle properties at the same time (see wave-
particle duality). Both wave and particle characteristics have been confirmed in a large number
of experiments. Wave characteristics are more apparent when EM radiation is measured over
relatively large timescales and over large distances while particle characteristics are more evident
when measuring small timescales and distances. For example, when electromagnetic radiation is
absorbed by matter, particle-like properties will be more obvious when the average number of
photons in the cube of the relevant wavelength is much smaller than 1. Upon absorption of light,
it is not too difficult to experimentally observe non-uniform deposition of energy. Strictly
speaking, however, this alone is not evidence of "particulate" behavior of light, rather it reflects
the quantum nature of matter.[1]
There are experiments in which the wave and particle natures of electromagnetic waves appear in
the same experiment, such as the self-interference of a single photon. True single-photon
experiments (in a quantum optical sense) can be done today in undergraduate-level labs.[2] When
a single photon is sent through an interferometer, it passes through both paths, interfering with
itself, as waves do, yet is detected by a photomultiplier or other sensitive detector only once.
