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The French physicist, Leon Foucault, 1819-1868. After training as a physician, Foucault turned to physics and joined Fizeau in measuring the speed of light. This measurement was improved when Foucault developed a method of reflecting light off mirrors. Foucault showed that light moves slower in water which was important evidence for the wave-theory of light. Foucault is most often associated with an experiment in 1850 to measure the Earth's rotation by swinging a giant pendulum (Foucault's pendulum). This was the first direct proof of the Earth's rotation. The gyroscope was invented by Foucault, and he discovered certain yellow (sodium D) lines in emission spectra.
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