The diffraction of a beam of electrons by a standing wave of light. This effect was predicted by Pyotr Kapitza (1894–1984) and Paul Dirac in 1933 as a consequence of the dual wave–particle nature of both electrons and light in quantum mechanics. It was not possible to verify the Kapitza–Dirac effect until the invention of lasers, with a clear demonstration of the effect being given in 2001. Since that time the Kapitza–Dirac effect has been used to investigate quantum-mechanical aspects of electrons and light.