Leon Foucault. Portrait of 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 he developed a method of reflecting light off mirrors. Foucault showed that light moves slower in water which was 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. He invented the gyroscope and discovered certain yellow (sodium D) lines in emission spectra.