Change in wavelength of X rays and other energetic forms of electromagnetic radiation when they collide with electrons. It is a principal way in which radiant energy is absorbed by matter, and is caused by the transfer of energy from photons to electrons. When photons collide with electrons that are free or loosely bound in atoms, they transfer some of their energy and momentum to the electrons, which then recoil. New photons of less energy and momentum, and hence longer wavelength, are produced; these scatter at various angles, depending on the amount of energy lost to the recoiling electrons. The effect demonstrates the nature of the photon as a true particle with both energy and momentum. Its discovery in 1922 by Arthur Compton was essential to establishing the wave-particle duality of electromagnetic radiation.
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