Compton Scattering

In the early 20th century, research into the interaction of X-rays with matter was an emerging field. It was observed that when a beam of X-rays is directed at an atom, an electron is ejected and the light is scattered through an angle θ, as illustrated below. Classical electromagnetism predicts that the wavelength of scattered rays should be equal to the initial wavelength; however, experiments showed that the wavelength of the scattered rays is actually greater than the initial wavelength. In 1923, Compton published a paper in the Physical Review explaining the phenomenon: A quantum theory of the scattering of X-rays by light elements (Phys. Rev. 21, 483 (1923)). Experimental measurements were also included in that paper, to support the theory.