Electromagnetic radiation (EM radiation or EMR) is a form of energy emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space. EMR has both electric and magnetic field components, which stand in a fixed ratio of intensity to each other, and which oscillate in phase perpendicular to each other and perpendicular to the direction of energy and wave propagation. In a vacuum, electromagnetic radiation propagates at a characteristic speed, the speed of light.
Electromagnetic radiation is a particular form of the more general electromagnetic field
(EM field), which is produced by moving charges. Electromagnetic
radiation is associated with EM fields that are far enough away from the
moving charges that produced them, that absorption of the EM radiation
no longer affects the behavior of these moving charges. These two types
or behaviors of EM field are sometimes referred to as the near and far field.
In this language, EMR is merely another name for the far-field. Charges
and currents directly produce the near-field. However, charges and
currents produce EMR only indirectly—rather, in EMR, both the magnetic
and electric fields are produced by changes in the other type of field,
not directly by charges and currents. This close relationship causes the
electric and magnetic fields in EMR to stand in a fixed ratio of
strengths to each other, and to be found in phase, with maxima and nodes
in each found at the same places in space.
