The Compton effect describes the increase in wavelength when light scatters from free or loosely bound electrons.
Arthur Holly Compton reported the effect in 1923 after directing X-rays at graphite and measuring the scattered radiation. The scattered X-rays had a longer wavelength than the incoming beam, and the change depended on the scattering angle.
Compton explained the result by treating electromagnetic radiation as particles carrying energy and momentum. In a collision, a photon transfers part of its energy and momentum to an electron, leaving the photon with lower energy and therefore a longer wavelength. The wavelength shift is given by the Compton formula and depends on the electron mass and scattering angle.
The effect provided powerful evidence for the photon concept. It is different from the Doppler effect, which arises from relative motion, and from atomic spectral-line shifts caused by electric or magnetic fields.