The Klein–Nishina formula gives the differential cross section of photons scattered from a single free electron in lowest order of quantum electrodynamics. At low frequencies this yields Thomson scattering; at higher frequencies this yields Compton scattering. For an incident unpolarized photon of energy, the differential cross section is: where is a differential cross section, is an infinitesimal solid angleelement, is the fine structure constant, is the scattering angle; is the "reduced" Compton wave length of the electron ; is the mass of an electron ; and is the ratio of photon energy after and before the collision: Note that this result may also be expressed in terms of the classical electron radius : While this classical quantity is not particularly relevant in quantum electrodynamics, it is easy to appreciate: in the forward direction, photons scatter off electrons as if these were about in linear dimension, and in size. If the incoming photon is polarized, the scattered photon is no longer isotropic with respect to the azimuthal angle. For a linearly polarized photon scattered with a free electron at rest, the differential cross section is instead given by: where is the azimuthal scattering angle. Note that the unpolarized differential cross section can be obtained by averaging over. The Klein–Nishina formula was derived in 1928 by Oskar Klein and Yoshio Nishina, and was one of the first results obtained from the study of quantum electrodynamics. Consideration of relativistic and quantum mechanical effects allowed development of an accurate equation for the scattering of radiation from a target electron. Before this derivation, the electron cross section had been classically derived by the British physicist and discoverer of the electron, J.J. Thomson. However, scattering experiments showed significant deviations from the results predicted by the Thomson cross section. Further scattering experiments agreed perfectly with the predictions of the Klein–Nishina formula. Note that if, and the Klein–Nishina formula reduces to the classical Thomson expression. The final energy of the scattered photon,, depends only on the scattering angle and the original photon energy, and so it can be computed without the use of the Klein–Nishina formula:
