Life of Léon Foucault, Physicist Who Measured the Speed of Light

Portrait of Leon Foucault
Portrait of Leon Foucault.

Public Domain

French physicist Léon Foucault played an essential role in measuring the speed of light and proving that Earth rotates on an axis. His scientific discoveries and contributions remain significant to this day, particularly in the field of astrophysics.

Fast Facts: Léon Foucault

  • Born: September 18, 1819 in Paris, France
  • Died: February 11, 1868 in Paris, France
  • Education: University of Paris
  • Occupation: Physicist
  • Known For: Measuring the speed of light and developing the Foucault pendulum (which proved Earth's rotation on an axis)

Early Life

Léon Foucault was born to a middle-class family in Paris on September 18, 1819. His father, a well-known publisher, died when his son was only nine years old. Foucault grew up in Paris with his mother. He was frail and often sick, and as a result he was educated at home until he entered medical school. He decided early on that he couldn't handle the sight of blood, and so left medicine behind to study physics.

During his work with mentor Hippolyte Fizeau, Foucault became fascinated with light and its properties. He was also intrigued by the new technology of photography being developed by Louis Daguerre. Eventually, Foucault began to study the Sun, learning about the physics of sunlight and comparing its spectrum to that of other light sources such as lamps. 

Scientific Career and Discoveries

Foucault developed experiments to measure the speed of light. Astronomers use the speed of light to determine the distances between objects in the universe. In 1850, Foucault used an instrument developed in partnership with Fizeau—now known as the Fizeau-Foucault apparatus —to prove that the once-popular "corpuscular theory" of light was not correct. His measurements helped establish that light travels slower in water than in air. Foucault continued improving his equipment to make ever-better measurements of light speed.

At the same time, Foucault was working on an instrument that became known as the Foucault pendulum, which he devised and installed at the Pantheon de Paris. The large pendulum is suspended overhead, swinging back and forth all day in a motion known as oscillation. As Earth rotates, the pendulum knocks over small objects placed in a circle on the floor underneath it. The fact that the pendulum knocks over these objects proves that Earth rotates on an axis. The objects on the floor spin with the Earth, but the pendulum suspended overhead does not.

Foucault was not the first scientist to build such a pendulum, but he brought the concept to prominence. Foucault pendulums exist in many museums to this day, providing a simple demonstration of our planet's spin.

Foucault Pendulum
The Foucault pendulum in the Pantheon de Paris. Public domain

Light continued to fascinate Foucault. He measured polarization (the geometry of light waves) and improved the shape of telescope mirrors in order to properly light. He also continued to strive to measure the speed of light with greater accuracy. In 1862, he determined that the sped was 298,000 kilometers per second. His calculations were quite close to what we know as the speed of light today: just under 300,000 kilometers per second. 

Later Life and Death

Foucault continued to carry out his experiments throughout the 1860s, but his health deteriorated. He developed muscular weakness and had difficulty breathing and moving, all signs of what could have been the degenerative disease multiple sclerosis. He was also reported to have suffered a stroke the year before his death. There have been some suggestions that he suffered from mercury poisoning after being exposed to the element during his experiments.

Léon Foucault died on February 11, 1868, and was buried in the Montmartre Cemetery. He is remembered for his wide-ranging and influential contributions to science, particularly in the field of astrophysics.


  • “Jean Bernard Léon Foucault.” Clavius Biography,
  • “Molecular Expressions: Science, Optics and You - Timeline - Jean-Bernard-Leon Foucault.” Molecular Expressions Cell Biology: Bacteria Cell Structure,
  • This Month in Physics History.
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Petersen, Carolyn Collins. "Life of Léon Foucault, Physicist Who Measured the Speed of Light." ThoughtCo, Aug. 28, 2020, Petersen, Carolyn Collins. (2020, August 28). Life of Léon Foucault, Physicist Who Measured the Speed of Light. Retrieved from Petersen, Carolyn Collins. "Life of Léon Foucault, Physicist Who Measured the Speed of Light." ThoughtCo. (accessed May 28, 2023).