Jean Bernard Léon Foucault
Léon Foucault was a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation.
Foucault was the son of a Parisian publisher from whom he received his early education. Later he studied medicine but discovered he was afraid of blood and abandoned that endeavor in favor of physics.
With A. H. L. Fizeau he carried out a series of investigations on the intensity of the light of the sun, as compared with that of carbon in the arc lamp, and of lime in the flame of the oxyhydrogen blowpipe; on the interference of infrared radiation, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light.
In 1850 Foucault conducted an experiment using the Fizeau–Foucault apparatus to measure the speed of light; it came to be known as the Foucault–Fizeau experiment, and was viewed as "driving the last nail in the coffin" of Newton's corpuscle theory of light by demonstrating that light travels more slowly through water than through air.
A year later he performed a conceptually simpler demonstration using a gyroscope, which he did not invent but is credited with naming. In 1855 Foucault received the Copley Medal of the Royal Society for his "very remarkable experimental researches". Earlier in the same year he was installed as physicien (physicist) at the imperial observatory at Paris.
In 1855 Foucault discovered that the force required for the rotation of a copper disc becomes greater when it is made to rotate with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy current or "Foucault currents" induced in the metal. In 1857 Foucault invented the polarizer which bears his name, and a year later devised a method of testing the mirror of a reflecting telescope to determine its shape. The "Foucault knife-edge test" allows the worker to tell if the mirror is perfectly spherical or has non-spherical deviation in its figure.
Prior to Foucault's publication of his findings the testing of reflecting telescope mirrors was a "hit or miss" proposition. Foucault's knife edge test determines the shape of a mirror by finding the focal lengths of its areas, commonly called zones and measured from the mirror center. The test focuses light point source at the center of curvature and reflected back to a knife edge. The test enables the tester to quantify the conic section of the mirror, thereby allowing the tester to validate the actual shape of the mirror, which is necessary to obtain optimal performance of the optical system. The Foucault test is still used today, most notably by amateur and smaller commercial telescope makers as it is inexpensive and uses simple, easily made equipment.
In 1862, using Charles Wheatstone’s revolving mirror, Foucault determined the speed of light to be 298,000 km/s (about 185,000 mi./s) —10,000 km/s less than that obtained by previous experimenters and only 0.6% off the currently accepted value.
That same year Foucault was inducted as a member of the Bureau des Longitudes and an officer of the Légion d'Honneur. In 1864 he was made a member of the Royal Society of London, and the next year a member of the mechanical section of the Institute.
In 1865 Foucault's papers on a modification of Watt's governor appeared. Foucault had for some time been experimenting with the generator hoping to make its period of revolution constant. He was simultaneously working on a new device for regulating the electric light, and showed how the sun could be viewed without injuring the eye.
Near the time of his death he returned to the Roman Catholicism that he previously abandoned. Foucault was buried in Paris in the Cimetière de Montmartre. The asteroid 5668 Foucault is named for him, and his name is one of the 72 inscribed on the Eiffel Tower.