An ingenious technique I'd not heard of before. Shoot a beam of light through a gap in a cogwheel, with a distant reflector set up to bounce it back through a different gap on the opposite side of the wheel when it's static. If the wheel is spinning fast enough, the returning light beam gets blocked by one of the teeth. Knowing the distance the light travels and the speed of the cogwheel, you can get the speed of light.
In 1849, Hippolyte Fizeau determined the speed of light in a famous experiment. The idea was to measure the time taken for a pulse of light to travel between an intense light source and a mirror about 8 km away. A rotating cogwheel with 720 notches, that could be rotated at a variable speed, was used to chop the light beam and determine the flight time. In 2017, physicists and technicians of the University of Mons in Belgium reproduced the experiment with modern devices to allow members of the public to measure the speed of light themselves. The light source used was a low power laser, and the cogwheel was replaced by an electrically driven chopper, but the general spirit of Fizeau's experiment was preserved. The exhibition was organised in the belfry of Mons, a baroque-style building classified as a UNESCO World Heritage site. The solutions found for the main problems encountered are presented here to help colleagues intending to reproduce the experiment.
https://arxiv.org/abs/1805.05762
I remember reading about the 1849 experiment when I was a boy. In the book it was right after a description of Galileo's failed attempt using two people on two mountains armed with shuttered oil lamps.
ReplyDeleteThen the book went on to talk about using the discrepancy between the calculated positions of the Jovian moons and their observed positions in order to deduce the speed of light.
ReplyDeleteThe same technique is used to determine the speed of a meteor as it passes through the atmosphere.
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