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 20 miles (35 kilometers) 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.
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