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Fizeau-Foucault apparatus

The Fizeau-Foucault apparatus (1850) was designed by the French physicists Hippolyte Fizeau and Léon Foucault for measuring the speed of light. The apparatus involves light reflecting off a rotating mirror, toward a stationary mirror some 20m away. As the rotating mirror will have moved slightly in the time it takes for the light to bounce off the stationary mirror (and return to the rotating mirror), it will thus be deflected away from the original source, by a small angle.

Foucault based his apparatus on an earlier experiment by Fizeau who, in 1849, used two fixed mirrors, one partially obscured by a rotating cogwheel. Fizeau's value for light's speed was about 5% too high.