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Michelson Interferometer
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In the Michelson interferometer light travels in two orthogonal directions, is reflected in two mirrors and returns to overlap at the origin. If both paths are equal and the light propagation is independent of the direction in which it travels, the beams will always interfere constructively.
The original experiment was created to study the eventual existence of an ether in space over which light would propagate. The existence of interference showed that there was no difference in the speed of propagation in the light in the direction of travel of the planet or perpendicular to it, implying that there would be no such ether and that the light propagates in a vacuum.
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