Compton scattering

Compton scattering (or the Compton effect) is the quantum theory of scattering of a high-frequency photon through an interaction with a charged particle, usually an electron. Specifically, when the photon interacts with a loosely bound electron, it releases the electron from an outer valence shell of an atom or molecule.

The effect was discovered in 1923 by Arthur Holly Compton while researching the scattering of X-rays by light elements, which earned him the Nobel Prize in Physics in 1927. The Compton effect significantly deviated from dominating classical theories, using both special relativity and quantum mechanics to explain the interaction between high frequency photons and charged particles.

Photons can interact with matter at the atomic level (e.g. photoelectric effect and Rayleigh scattering), at the nucleus, or with only an electron. Pair production and the Compton effect occur at the level of the electron.^[1] When a high-frequency photon scatters due to an interaction with a charged particle, the photon's energy is reduced, and thus its wavelength is increased. This trade-off between wavelength and energy in response to the collision is the Compton effect. Because of conservation of energy, the energy that is lost by the photon is transferred to the recoiling particle (such an electron would be called a "Compton recoil electron").

This implies that if the recoiling particle initially carried more energy than the photon has, the reverse would occur. This is known as inverse Compton scattering, in which the scattered photon increases in energy.