Jean Bernard Léon Foucault was a French physicist born on September 18, 1819, and died on February 11, 1868. He invented the Foucault pendulum, a tool that shows how the Earth rotates. He also measured the speed of light early in his career. Additionally, he discovered eddy currents and gave the gyroscope its name.

Early years

Foucault was born on September 18, 1819, in Paris, France. He was the son of a publisher and received most of his early education at home. He initially studied medicine but later chose to study physics because he had a fear of blood. He first focused on improving the photographic methods developed by Louis Daguerre. For three years, he worked as an assistant to Alfred Donné (1801–1878) during Donné’s lectures on microscopic anatomy.

With Hippolyte Fizeau, Foucault conducted experiments comparing the brightness of sunlight to the light produced by carbon in an arc lamp and lime in the flame of an oxyhydrogen blowpipe. He also studied how infrared light interacts and how light rays that travel different distances interfere with each other. Additionally, he examined how the color of light affects its polarization.

In 1849, Foucault proved through experiments that absorption and emission lines that appear at the same wavelength are caused by the same material. The difference between these lines is due to the temperature of the light source.

Middle years

In 1850, he conducted an experiment using a spinning mirror to measure the speed of light. This showed that light moves more slowly through water than through air, which was seen as proving that Newton’s theory that light is made of tiny particles was incorrect. In 1851, he demonstrated the Earth’s rotation around its axis by using a long, heavy pendulum hanging from the roof of the Panthéon in Paris. The pendulum’s swinging motion changed over time, showing the Earth’s movement. This experiment became famous and led to the creation of similar pendulums in cities across Europe and America. In 1852, he used a device called a gyroscope to provide a simpler way to show Earth’s rotation. In 1855, he was honored with the Copley Medal by the Royal Society for his important scientific research. Earlier that year, he was appointed as a physicist at the Paris imperial observatory.

In September 1855, he discovered that when a copper disc spins inside a magnet’s poles, more force is needed to keep it moving. At the same time, the disc heats up due to circular electric currents, later called “Foucault currents.”

In 1857, he created a device called a polarizer, which is still named after him. The next year, he developed a method to test the shape of mirrors in reflecting telescopes. His method, called the “Foucault knife-edge test,” uses light from a single source focused on the mirror’s center. The light is then reflected back toward a sharp edge, allowing the tester to see if the mirror is perfectly curved or has imperfections. Before this method, testing telescope mirrors was often done by trial and error.

The Foucault knife-edge test works by measuring the focal points of different parts of the mirror, called zones. By analyzing how light bends, the tester can determine the mirror’s exact shape, which is needed for the telescope to work well. This test is still used today, especially by amateur telescope makers, because it is simple and inexpensive.

In 1862, he worked with Charles Wheatstone to measure the speed of light using a spinning mirror. Their result was about 298,000 km/s, which was 10,000 km/s slower than earlier measurements but only 0.6% off the current accepted value.

Later years

In 1862, Foucault was made a member of the Bureau des Longitudes and awarded the Legion of Honour. In 1864, he became a member of the Royal Society of London, and the following year, he joined the mechanical section of the Institute. In 1865, he published papers about an improvement to James Watt's centrifugal governor. For many years, he had been experimenting to make the governor's rotation speed stay constant and to create a new device for controlling electric light. Foucault demonstrated that applying a very thin layer of silver to the outer side of a telescope's lens allowed the sun to be viewed safely without harming the eyes. His main scientific papers are found in the Comptes Rendus from 1847 to 1869. Close to his death, he returned to Roman Catholicism, which he had left earlier.

Death and legacy

Foucault died from an illness that was likely multiple sclerosis on February 11, 1868, in Paris. He was buried in Montmartre Cemetery. The asteroid named 5668 Foucault was named after him. His name is one of the 72 names written on the Eiffel Tower.