The reduction in the energy of high-energy (X-ray or gamma-ray) photons when they are scattered by free electrons, which thereby gain energy. The phenomenon, first observed in 1923 by A. H. Compton, occurs when the photon collides with an electron; some of the photon's energy is transferred to the electron and consequently the photon loses energy h(ν1 – ν2), where h is the Planck constant and ν1 and ν2 are the frequencies before and after collision. As ν1 > ν2, the wavelength of the radiation increases after the collision. This type of inelastic scattering is known as Compton scattering and is similar to the Raman effect. See also inverse Compton effect.