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Fizeau used a rotating toothed wheel to measure the speed of light. A rough sketch of the apparatus is given below.
By spinning the wheel, Fizeau could find the time it took the light to travel from the wheel to the mirror and back to the wheel. He achieved this by increasing the speed of the wheel until the reflection from the mirror across the room disappeared in his viewing piece. He knew that when the reflection disappeared a tooth had rotated just enough to block the returning pulse of light from the mirror. He then could calculate a time by using basic rotational motion equations.
Created 8/14/2009