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Kapitsa–Dirac effect
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<p>The Kapitza–Dirac effect is a <a href="/facts/Quantum_mechanics/BRc3Mzgr">quantum mechanical</a> effect consisting of the diffraction of matter by a <a href="/facts/Standing_wave/n134E8TC">standing wave</a> of light, in complete analogy to the diffraction of light by a periodic grating, but with the role of matter and light reversed. The effect was first predicted as the <a href="/facts/Electron_diffraction/IlFzQyVu">diffraction of electrons</a> from a standing wave of light by <a href="/facts/Paul_Dirac/6bt5yRdk">Paul Dirac</a> and <a href="/facts/Pyotr_Kapitsa/xhHdzHBd">Pyotr Kapitsa</a> (or Peter Kapitza) in 1933. The effect relies on the wave–particle duality of matter as stated by the <a href="/facts/Matter_wave/Rpszb8f1">de Broglie hypothesis</a> in 1924. The matter-wave diffraction by a standing wave of light was first observed using a beam of neutral atoms. Later, the Kapitza-Dirac effect as originally proposed was observed in 2001. </p>