Fizeau's method

Fizeau's method measures the speed of light by passing a beam through a rotating toothed wheel. As the wheel spins, the teeth intermittently block and allow the light to travel to a reflector some distance away and back. By adjusting the wheel's speed, scientists find a point where the returning light is blocked when a tooth is in front of the opening, and passes when a gap aligns. From this, they calculate the light's speed based on the wheel's rate of rotation and the distance between the wheel and reflector.