## Fizeau's Rotating Wheel Method for Measuring the Speed of Light

Fizeau's rotating wheel experiment was first performed in 1849, and was the first experiment to measure the speed of light that did not require any but terrestrial observations. The apparatus is shown below. Light from a source  is focused onto a rotating toothed wheel by reflection from a glass slide. Light passes through a gap between the teeth of the wheel. The reflected light is focused by mirrors andonto a mirror M, then back onto the toothed wheel. The speed of rotation of the wheel is adjusted so that the time for the light to pass from the wheel to M and back is the same as the time taken for a tooth to move into the gap through which the light passed.

If the distance from the wheel to M isthen in the time taken for a tooth to move into the gap, the light has travelled a distanceIf the wheel hasteeth, radiusand is rotating at revolutions per second, then the tooth has travelled a fractionof the circumference of the wheel at a speed ofso

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