Fizeau , Armand Hippolyte Louis

(1819–1896) French physicist

Fizeau, a Parisian by birth, started by studying medicine but his interest turned to optics before he finished the course. In collaboration with Léon Foucault he first tried to improve the newly developed process of photography and, in 1845, they took the first clear pictures of the Sun.

In 1849 he obtained a value for the speed of light in air, using an ingenious toothed-wheel apparatus. Light was directed through a gap between two teeth and reflected back between the teeth from a distant mirror. The wheel was rotated, the rate of rotation being changed until the reflected flashes were blocked by the tooth of the wheel. The speed of light could then be calculated from the rate of rotation of the wheel. Fizeau's experiment was performed using a path of 8 kilometers (5 mi) between Suresnes and Montmartre.

The next year both he and Foucault simultaneously proved that light traveled faster in air than in water, thus giving experimental support to the wave theory of light. Fizeau is also known for analyzing the Doppler effect for light waves. The change in wavelength with relative speed is sometimes called the Doppler–Fizeau shift.

Fizeau was elected a member of the Paris Academy in 1860 and was awarded the Royal Society's Rumford medal in 1875.