Fizeau's experiment

Fizeau's experiment, conducted in 1849, measured the speed of light using a beam of light reflected off a moving mirror. The light beam was split, sent toward a rotating wheel with teeth, and reflected back. As the wheel rotated, its teeth would intermittently block or allow the light to pass. By adjusting the wheel’s speed, Fizeau determined the point where the light was fully blocked or allowed through, enabling him to calculate the light's finite speed. This experiment provided one of the first terrestrial measurements confirming that light travels at a constant, finite speed.