Abstract
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.
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