Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as [[retarded time]] (as opposed to advanced time, which is unphysical in light of [[causality]]), which adds to the expressions for the electrodynamic [[electric field]] and [[magnetic field]]. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an [[antenna (electronics)|antenna]]) conducts [[alternating current]], electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a [[wave]] or as [[photon|particle]]s. As a wave, it is characterized by a velocity (the [[speed of light]]), [[wavelength]], and [[frequency]]. When considered as particles, they are known as [[photon]]s, and each has an energy related to the frequency of the wave given by [[Max Planck|Planck's]] relation ''E = hν'', where ''E'' is the energy of the photon, ''h'' = 6.626 × 10<sup>-34</sup> J·s is [[Planck's constant]], and ''ν'' is the frequency of the wave.
One rule is always obeyed regardless of the circumstances: EM radiation in a vacuum always travels at the [[speed of light]], ''relative to the observer'', regardless of the observer's velocity. (This observation led to [[Albert Einstein]]'s development of the theory of [[special relativity]].)
In a medium (other than vacuum), [[velocity of propagation]] or [[refractive index]] are considered, depending on frequency and application. Both of these are ratios of the speed in a medium to speed in a vacuum.
== Spéktrum éléktromagnétik ==