From 1848 to 1849,

Hippolyte Fizeau Armand Hippolyte Louis Fizeau (; 23 September 1819 – 18 September 1896) was a French physicist who, in 1849, measured the speed of light to within 5% accuracy. In 1851, he measured the speed of light in moving water in an experiment known as t ...

used a toothed wheel apparatus to perform absolute measurements of the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

in air. Subsequent experiments performed by

Marie Alfred Cornu Marie Alfred Cornu (; 6 March 1841 – 12 April 1902) was a French physicist and professor of École polytechnique. The French generally refer to him as Alfred Cornu. The Cornu spiral, a graphical device for the computation of light intensities ...

from 1872 to 1876 improved the methodology and made more accurate measurements.

Fizeau's determination of the speed of light

In 1848–49,

determined the speed of light using an intense light source at the bell tower of his father's holiday home in

Suresnes Suresnes () is a commune in the western inner suburbs of Paris, France. Located in Hauts-de-Seine, from the centre of Paris, it had a population of 49,482 as of 2020. Suresnes borders the Bois de Boulogne in the 16th arrondissement of Paris, ...

, and a mirror 8,633 meters away on

Montmartre Montmartre ( , , ) is a large hill in Paris's northern 18th arrondissement of Paris, 18th arrondissement. It is high and gives its name to the surrounding district, part of the Rive Droite, Right Bank. Montmartre is primarily known for its a ...

. The light source was interrupted by a rotating cogwheel with 720 notches that could be rotated at a variable speed several times a second. (Figure 1) Fizeau increased the rotation speed of the cogwheel until light passing through one notch of the cogwheel would be completely eclipsed by the adjacent tooth. At 12.6 rotations per second, the light was eclipsed. At twice this speed (25.2 rotations per second), it was again visible as it passed through the next notch. At 3 times the speed it was again eclipsed. Given the rotational speed of the wheel and the distance between the wheel and the mirror, Fizeau was able to calculate a value of for the speed of light. Fizeau's value for the speed of light was 4.5% too high. The correct value is 299,792,458 m/s. It was difficult for Fizeau to visually estimate the intensity minimum of the light being blocked by the adjacent teeth. Other sources of error include the measurement of the distance from the wheel to the mirror, and the measurement of the speed of rotation of the wheel. Fizeau's paper appeared in ''Comptes Rendus Hebdomadaires de séances de l’Academie de Sciences'' (Paris, Vol. 29 uly–December 1849 pp. 90–92).

Cornu's refinement of the Fizeau experiment

At the behest of the

Paris Observatory The Paris Observatory (, ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world. Its historic building is on the Left Ban ...

under

Urbain Le Verrier Urbain Jean Joseph Le Verrier (; 11 March 1811 – 23 September 1877) was a French astronomer and mathematician who specialized in celestial mechanics and is best known for predicting the existence and position of Neptune using only mathematics. ...

repeated Fizeau's 1848 toothed wheel measurement in a series of experiments from 1872 to 1876. The goal was to obtain a value for the speed of light accurate to one part in a thousand. Cornu's equipment allowed him to monitor high orders of extinction, up to the 21st order. Instead of estimating the intensity minimum of the light being blocked by the adjacent teeth, a relatively inaccurate procedure, Cornu made pairs of observations on either side of the intensity minima, averaging the values obtained with the wheel spun clockwise and counterclockwise. An electric circuit recorded the wheel rotations on a chronograph chart, which enabled precise rate comparisons against the observatory clock. A telegraph key arrangement allowed Cornu to mark the precise moments when he judged that extinction had been entered on this same chart or exited. His final experiment was run over a path nearly three times as long as that used by Fizeau. This experiment yielded a figure of 300,400,000 m/s, which is 0.2% above the actual value.

References

External links

Sur une experience relative a la vitesse de propagation de la lumière by H. Fizeau (1849)

A modern Fizeau experiment for education and outreach purposes
*{{ YouTube , title= How to find the speed of light (Fizeau experiment) , id= a6gl8KZM0PM , very fascinating animation, which makes everything immediately clear Optical metrology Physics experiments

Fizeau's determination of the speed of light

Cornu's refinement of the Fizeau experiment

See also

References

External links