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The double-slit experiment demonstrates wave-particle duality: individual quantum particles fired one at a time still build up an interference pattern, as if each particle passes through both slits simultaneously as a wave and interferes with itself. Thomas Young first performed the experiment with light in 1801, using it to argue convincingly that light behaves as a wave and measuring the wavelength of visible light from the fringe spacing. In the twentieth century the experiment was repeated with electrons, neutrons, and even large molecules, always producing the same fringes when both paths are left open. The moment a detector determines which slit a particle actually went through ("which-path" information), the interference pattern collapses into two simple bands matching classical single-slit diffraction — measurement itself changes the outcome. Coherence between the two paths is essential: the light or particle source must be coherent enough that the phase relationship between the two slit paths stays stable long enough to interfere.

• Visible light wavelength ranges roughly 400–700 nm; violet is shortest, red is longest.
• Classic optical double-slit setups use slit widths of a few micrometers and separations of tens to hundreds of micrometers.
• Fringe spacing on the screen scales as λL/d, where L is the slit-to-screen distance and d is the slit separation.
• Electron double-slit experiments (e.g. Tonomura, 1989) used electrons one at a time, each landing as a single dot that only formed fringes after thousands of hits.
• Closing one slit destroys interference and leaves a single-slit diffraction envelope (a broad central peak with weak side lobes).
• "Which-path" detection at a slit destroys the interference pattern even without disturbing the particle's momentum much, illustrating complementarity.
• Young's original 1801 experiment used sunlight through a pinhole and card to split it into two coherent sources.