Compton effect [for A. H. Compton], increase in the wavelengths of X rays and gamma rays when they collide with and are scattered from loosely bound electrons in matter. This effect provides strong verification of the quantum theory since the theoretical explanation of the effect requires that one treat the X rays and gamma rays as particles or photons (quanta of energy) rather than as waves. The classical treatment of these rays as waves would predict no such effect. According to the quantum theory a photon can transfer part of its energy and linear momentum to a loosely bound electron in a collision. Since the energy and magnitude of linear momentum of a photon are proportional to its frequency, after the collision the photon has a lower frequency and thus a longer wavelength. The increase in the wavelength does not depend upon the wavelength of the incident rays or upon the target material. It depends only upon the angle that is formed between the incident and scattered rays. A larger scattering angle will yield a larger increase in wavelength. The effect was discovered in 1923. It is used in the study of electrons in matter and in the production of variable energy gamma-ray beams.
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