Collision between a photon and an atomic electron
The Compton effect occurs for most of the atomic electrons. A gamma photon plays the role of a projectile that collides with an electron in an atom that serves as a target. Gamma was represented as a punctual particle because of its very short wavelength at the atomic scale. As the vast majority of electrons posses a smaller energy than the one of gamma, physicists are accustomed to neglect it and to consider the electron as a target at rest. In the collision, the electron is put in motion at a certain angle, while the gamma scattered with another angle loses its energy.