What did the double-slit experiment show?

The double-slit experiment is an important physics demonstration of how light and matter can exhibit behavior associated with both classical particles and classical waves. In the experiment, the wave nature of light causes the light waves passing through two slits to interfere, producing bright and dark bands on the screen – a result that would not be expected if light consisted of classical particles. However, the light is always found to be absorbed at the screen at discrete points, as individual particles (not waves). The more mind-boggling part? When scientists placed detectors to determine which slit an electron passed through (as electrons exhibit the same effects as photons) the interference pattern disappears. This “which way” experiment demonstrated the concept in quantum mechanics called wave-particle duality, in which photons or electrons exhibit particle or wave properties according to the experimental circumstances.

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