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Armand-Hippolyte-Louis Fizeau, (b. Sept. 23, 1819, Paris--d. Sept. 18, 1896, Nanteuil-le-Haudouin, Fr.), French physicist noted for his experimental determination of the speed of light. Fizeau worked with Jean-Bernard-Léon Foucault on investigations of the infrared portion of the solar spectrum and made other observations of heat and light.

Unaware of Christian Doppler's publication (1842), Fizeau in 1848 gave an explanation of the shift in wavelength in light coming from a star and showed how it could be used to measure the relative velocities of stars that lie in the same line of sight. In 1849 Fizeau found the first reasonably accurate value of the velocity of light obtained in a nonastronomical experiment.

In 1851 he carried out a series of experiments in an attempt to detect the luminiferous ether--a hypothetical material that was thought to occupy all of space and to be necessary for carrying the vibrations of light waves. The experimental results failed to demonstrate the existence of the ether, but his work helped lead to the discarding of the ether theory in the early years of the 20th century.

Fizeau became a member of the French Academy in 1860 and was appointed superintendent of physics at the École Polytechnique, Paris, in 1863.

Source: http://www.eb.com:180/cgi-bin/g?DocF=micro/210/86.html

Armand-Hippolyte-Louis Fizeau (1819 - 1896), 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 Leon Foucault between 1839 and 1847. 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.

Source: Random House Webster's Dictionary of Scientists

Fizeau, Armand Hippolyte Louis (1819-1896) French physicist, born in Paris into a wealthy family. His father was Professor of Internal Pathology at the Paris Faculty of Medicine. After interrupting his medical studies because of poor health, he moved into the physical sciences, and was greatly influenced by the lectures of Arago. During the early period of his research he collaborated with Foucault, another medical student who decided on a career in physics. They met while Fizeau was working on improving the Daguerreotype process, and were the first to obtain a detailed photographic image of the Sun (1845).

In 1849 Fizeau was the first to measure the velocity of light by a laboratory experiment in which a ray of light was cut by a toothed wheel, producing intermittent flashes. The velocity of light could then be calculated from the speed of the rotation and the distance. His figure (about 315000 kilometres per second) was not as accurate as that produced by astronomical calculations, but he showed the practicability of this approach, which was then improved by others, notably by Alfred Cornu in the 1870s. Fizeau's adaptation of this technique to measure the velocity of electricity in a wire was less successful.

He also demonstrated the use of the shift in light frequency (the 'redshift') in determining a star's velocity along the line of sight. Unknown to him, Doppler had already published this effect, but had not fully understood the implications. The British astronomer William Huggins was the first to determine the velocity of a star relative to the Earth by this method (1868).

Source: Larousse Dictionary of Scientists

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