Armand Hippolyte Louis Fizeau (23.09.1819 — 18.09.1896) —a French physicist, a member of the Paris Academy of Sciences (1860) and its president (1878). He was born in Paris. He studied at the Collège de France and the Paris Observatory. From 1863 he was professor at the École Polytechnique.
His main work concerned optics. In 1849 he developed a method of determining the speed of light by means of a rotating toothed wheel (the Fizeau method) and was the first to measure the speed of light under terrestrial conditions. He established the influence of the motion of a medium on the speed of propagation of light by measuring in 1851 the speed of light in moving water (the Fizeau experiment). With this experiment he proved that light is partly dragged along by a moving medium. The Fizeau experiment played an important role in establishing the wave theory of light, and also in the formation and testing of the basic equations of the electrodynamics of moving media; in particular, the relativistic formula for the addition of velocities received confirmation in Fizeau's experiments. In 1848 he gave the correct interpretation of the Doppler effect in optics, showing that when a source of light and an observer move relative to each other the wavelength of the light must change (the Doppler — Fizeau effect). Proceeding from this, he proposed applying the Doppler principle to determine the radial velocity of celestial bodies from the shift of the spectral lines in their spectra. Together with L. Foucault he detected (1844 — 47) the Fraunhofer lines in the infrared region of the spectrum. He developed a method of observing the interference of light at large path differences (1849) and an interference method (1852) of measuring the coefficient of expansion of solids and the angular diameter of stars.
He established that, besides water, other substances have a density maximum. He constructed an induction coil, an interference spectroscope and a dilatometer, studied crystals, and took up photography.
A Fellow of the Royal Society (1875). The B. Rumford Medal (1866).