photon (fo´tòn´) or light quantum, the particle composing light and other forms of electromagnetic radiation. According to Einstein's special theory of relativity, the speed of light (c=2.998 x 10⁸ m/sec) always appears the same, regardless of the motion of the observer, but the motion of material objects is relative to the motion of the observer. Light travels at different speeds in different media. The photoelectric effect and blackbody radiation can be explained only by assuming that light energy is transferred in discrete packets, or photons, and that the energy of each photon is equal to the frequency of the light multiplied by Planck's constant (h=6.6262 x 10^-34 joule·sec).

E = h·f

Light imparts energy to a charged particle when one of its photons collides with the particle. See also quantum mechanics.

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