# Compton effect

A.H. Compton presented another evidence of the quantum theory, or photon theory, in 1923. In 1927, he was awarded the Nobel Prize for his discovery of the Compton Effect. When X-rays of wavelength’ contacted a piece of graphite, one electron was expelled, and the X-rays scattered at an angle had a longer wavelength, he proved.

## Explanation of Compton Effect

Compton compared it to a ball striking a stationary ball, which is pushed away as the striking ball’s energy drops. As a result, he claimed that light radiation (X-rays) was made up of particles (photons) because the electron could not have been thrown out by a continuous wave. He imagined that a photon of incoming light collided with a stationary electron in graphite, losing some energy and so increasing the wavelength. Unless light radiation had particles or photons, this process could not have occurred.

By assuming photon-electron collisions to be perfectly elastic, Compton found that the shift in wavelength, dλ was given by the expression

where h is Planck’s constant, m is the mass of an electron, c the velocity of light, and θ the angle of scattering. The expression shows that dλ is independent of the nature of the substance and the wavelength of the incident radiation. Given the wavelength of a photon, one can calculate the momentum of the electron ejected.