The Compton Effect is a principal way in which radiant energy is absorbed by matter, and is caused by the transfer of energy from photons to electrons.  When photons collide with electrons that are free or loosely bound in atoms, they transfer some of their energy and momentum to the electrons, which then recoil.  New photons are created with momentum and energy, which is why they have a longer wavelength, are produced; these scatter at various angles, depending on the amount of energy lost to the recoiling electrons.  The Compton Effect demonstrates the nature of the photon as a true particle with both energy and momentu.  Its discovery in 1922 by Arthur Compton was essential to establishing the wave-particle duality of electromagnetic radiation.