PhysicsLAB: Famous Experiments: The Compton Effect

A. H. Compton discovered that x-ray photons when collided with electrons in a graphite crystal displayed particle properties of momentum equal to

Compton's equation for the photon's wavelength after its collision with the electron was

where θ is the angular deviation of the x-rays from their original path.

Notice that when the x-rays emerged from the crystal they have a wavelength longer than their original value. This lengthening of their wavelength showed that they had lost energy. The lost energy was accounted for in the KE of an ejected electron from the crystal.

Derivation of the expression for the momentum of a photon

from special relativity

using the definition for the momentum of a particle and substituting for its mass

using the definitions for the energy of a photon and the frequency of a wave

In 1927, Arthur H. Compton was awarded the Nobel Prize in Physics for his discovery of the particle properties of x-rays.