Louis Fizeau - speed of light measured using a rotating toothed wheel
To measure the speed of light using the lanterns Galileo suggested was just not practical enough. If the two people were ten miles apart, they would have been able to detect the time lag of one-ten thousandth of a second!!! (They also did not have electronic stop watches back then. They probably used buckets of water to measure time intervals.)
A more practical way had to be found to measure this very, very short time lag.
Two rivals, Fizeau and Foucault independently solved this problem in France.
He shone a light between the teeth of a rapidly rotating toothed wheel. A mirror reflected the beam back between the same gap between the teeth of the wheel.
There were over a hundred teeth in the wheel. The wheel rotated at hundreds of times a second - therefor thousands of a second was easy to measure. Light was reflected from mirrors more than 5 miles apart. This also helped him making accurate measurements.
By varying the speed of the wheel is was possible to determine at what speed the wheel was spinning too fast for the light to pass through the gap between the teeth and back through the same gap.
Fizeau calculated the speed of light to be 313,300 Km/sec. (He knew how short a time the light had to get through that gap and back, and he knew how far the light travelled. By dividing the distance by the time he got the speed of light.)
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