Compton Effect was a key experiment that provided the most direct evidence for the particle nature of radiation. Arthur Holly Compton, an American Physicist discovered that, radiation of a given wavelength (in the X-ray region) sent through a metallic foil was scattered in a manner not consistent with classical theory. As per classical theory, the mechanism for the effect is the re-radiation of light by electrons set into forced oscillations by the incident radiation, and this leads to the prediction of intensity observed at an angle ϴ that varies as (1 + cos2ϴ) and does not depend on the wavelength of the incident radiation. Also, according to classical theory, the frequency or wavelength of the scattered radiation should depend on the length of time the electron was exposed to the incident radiation as well as the intensity of the incident radiation.
Compton found that the radiation scattered through a given angle actually consists of two components: one whose wavelength is the same as that of the incident radiation, the other wavelength shifted relative to the incident wavelength by an amount that depends on the angle. In other words, Compton showed experimentally that the wavelength shift of x-rays scattered at a given angle is absolutely independent of the intensity of radiation and the length of exposure and depends only on the scattering angle.
Related: Black Body Radiation
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