Hippolyte Fizeau

Affiliations: 
École Polytechnique, Palaiseau, Île-de-France, France 
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"Hippolyte Fizeau"
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Fizeau is known for his measurements of the speed of light. He independently discovered the Doppler-effect.
In 1849 Fizeau, working closely with Foucault, directly measured the speed of light on the Earth. Fizeau used a cogwheel and a mirror located several miles apart, which were set up to allow a pulsing light beam to pass between them. By rotating the cogwheel, Fizeau was able to observe the light beam passing between the cogs of the wheel to the distant mirror and then reflected back. If he spun the wheel fast enough, he was able to obscure the reflection. This meant that the reflected light beam struck a cog. Fizeau proposed that the time it took for the wheel to move the width of one cog must be equal to the time it takes for the light beam to travel to the mirror and back to strike the cog. Knowing the rotational speed of the cogwheel, the width of one cog, and the distance to the mirror, were all that was required for Fizeau to make the simple calculation of the speed of light. His calculation was very close to the current accepted value of the speed of light in a vacuum of 186,282 miles per second.

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Mean distance: 12.4 (cluster 21)
 
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