To understand some important aspects of electromagnetic field behavior, it will simplify matters to think for a moment about an electron traveling freely through space. We generally say it has a certain amount of energy, which is proportional to its velocity and mass. But the electron is a creature of the electromagnetic field; interaction with the fields around it may change its velocity and thus add or subtract energy. Its losses and gains are ripples in the magnetic field. A slowing electron sheds some energy; the loss is a change in the field around it that propagates outward as a wave. The electron might absorb a wave, too, boosting its energy.

If such a wave were drawn schematically, it must have a certain height (amplitude) and breadth (wavelength). These are proportional to the energy changes involved. The wave is, in fact, a packet of purest energy. This is the way energy is exchanged in the electromagnetic field. Quantum theory advises, however, that waves and particles are not distinct things. As a rule of thumb, small-wavelength packets behave more in the way we expect particles should, the longer lengths act more wavelike. The particle-name for electromagnetic energy is photon—though in everyday usage the word is most likely applied to just those wavelengths we perceive as light. Of wave-names we have a larger supply (depending on the range of wavelengths included): radio, microwave, infrared, x-ray, gamma ray, and others. All, however, refer to the same physical phenomenon, some increment of energy emitted or absorbed by a charged particle.