- French physicist
who in 1849 was the first to measure the speed of light on the Earth's
surface. He also found that light travels faster in air than in water,
which confirmed the wave theory of light, and that the motion of a star
affects the position of the lines in its spectrum.
Fizeau, born in Paris, studied at the College de France and with François
Arago at the Paris Observatory. Many of his discoveries were made in
collaboration with Léon Foucault 1839-47.
Fizeau began to research into the new science of photography in 1839,
and with Foucault developed daguerreotype photography for astronomical
observations by taking the first detailed pictures of the Sun's surface
1845. They also found, in 1847, that heat rays from the Sun undergo
interference and that radiant heat therefore behaves as a wave motion.
To determine the speed of light, Fizeau sent a beam through the gaps
in the teeth of a rapidly rotating cog wheel to a mirror 8 km/5 mi away.
On returning, the beam was brought to the edge of the wheel, the speed
being adjusted so that the light was obscured. This meant that light
rays which had passed through the gaps were being blocked on their return
by the adjacent teeth as they moved into the position of the gaps. The
time taken for the teeth to move this distance was equal to the time
taken for light travel 16 km/10 mi to the mirror and back.