The scattering of photons (of gamma or X rays) by the orbital electrons of atoms. In a collision between a (primary) photon and an electron, some of the energy of the photon is transferred to the electron which is generally ejected from the atom. Another (secondary) photon, with less energy, then moves off in a new direction at an angle to the direction of motion of the primary photon.

An interaction between a photon and an electron, in which the photon scatters off the electron, as in a collision between billiard balls, and comes off with less energy. The effect provides a convincing demonstration of the quantization of light energy.

(Also Compton scattering.) Scattering of x-rays and gamma radiation by matter in which the frequency of the scattered radiation is measurably less than that of the incident radiation (inelastic scattering). So named because Arthur Compton was the first to explain the observed frequency shift by applying the laws of energy and momentum conservation to scattering of a photon by a free electron. Compton's experimental and theoretical investigations established the validity of the quantum theory of radiation, showing that photons possess momentum (and hence can exert radiation pressure) as well as energy. Boorse, H. A., and L. Motz, 1966: The World of the Atom, Vol. II, 902–929.