Foucault - speed of light measured using a rotating mirror
Leon Foucault bounced light from a rotating mirror back to a stationary mirror. The light from the rotating mirror bounced back at an angle slightly different from the angle it hit the mirror with - because the mirror was rotating. By measuring this angle it was possible to measure the speed of the light.
Foucault continually increased the accuracy of this method over the next 50 years. His final measurement in 1926 determined that light travelled at 299,796 Km/s.
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