The Compton effect or Compton scattering is when a free charged particle usually an electron emits photons. This results in an increase in wavelength of the photon (which may be an X Ray or a Gamma Ray photon). Part of the energy of the photon is transferred to the recoiling electron. The opposite can also happen - known as inverse Compton scattering, which involves a charged particle (i.e. electron) transferring its energy to the photon.