Fizeau, Armand Hippolyte Louis (1819–1896)

French physicist. In 1849 he 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.

Using photography Fizeau began to research into the new science of photography in 1839, and with Léon 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.

Speed of light 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 to travel 16 km/10 mi to the mirror and back.

Life 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.

Red shift Fizeau may have been unaware of the effect discovered 1842 by the Austrian physicist C J Doppler when, in 1848, he suggested that a moving light source such as a star undergoes a change in observed frequency that can be detected by a red–blue shift in its spectral lines. Fizeau's discovery is now the basis of the principal method of determining the distances of galaxies and other distant bodies. Fizeau also determined the amount of drift of light waves in a transparent medium that is in motion.