The Compton effect is a well known phenomenon discovered in 1923 by the Nobel physics prize winner Arthur Compton showing that light is made up of quanta of energy having momentum. The effect occurs on the collision of an energetic photon, such as an X-ray photon, with a particle of matter, such as an electron, considered to be at rest.
The Nobel physics prize winner Arthur Compton. © The Nobel Foundation
This collision is accompanied by a change in wavelength of the photon after the collision which is very similar to a collision between two particles. When the photon has a longer wavelength following the collision, i.e. it has lost energy to the electron, Compton scattering has occurred.
When the wavelength of the photon is shorter and therefore the photon has gained energy from the electron, inverse Compton scattering has occurred. This is a particularly important effect in astrophysics and cosmology since it explains the Sunyæv-Zel'dovich effect.
A diagram illustrating Compton scattering, i.e. the scattering of a photon following collision with a particle, here an electron. The formula gives the final photon wavelength as a function of its initial wavelength and the scattering angle theta. © Robert Hart
Compton scattering - 2 Photos