The double slit experiment involves passing light through two parallel slits to demonstrate that the light from one slit interferes with that from the other.
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The double slit experiment, also called Young's double slit experiment after Thomas Young (1773–1829) who first performed it, passes light through two small parallel slits. Behind the slits is a flat surface. The light that hits the surface is "banded", with alternating series of lighter and darker sections. This pattern is called an interference pattern.
This pattern corresponds to the wavelength of the light. In the center of the pattern, the light rays from both slits travel the same distance to hit the surface, and the two rays reinforce each other to produce a bright spot. At a neighboring light section, the light from one slit travels one wavelength farther than the other, and the rays again reinforce each other. In between, in the dark spot, the rays interfere with each other, essentially canceling each other out.
More information is available on Wikipedia.