© Left: NASA, Right: David Kaplan.
Arthur Holly Compton (left), a physicist at Washington University in St. Louis, MO, did a careful measurement of light scattering off a crystal in 1923. He found that the frequency of light was related to the scattering angle: The frequency of scattered light decreased more when the light was scattered through larger angles. He explained this through the interaction of light (photons) with electrons in the crystal. On the right is a Feynman diagram representing a part of the calculation of Compton scattering. In the diagram, time increases to the right. An electron (blue) and a photon (red) interact in a way that can change the final energy and momentum of each particle. (Unit: 2)