Léon Foucault

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Léon Foucault
Léon Foucault (1819–1868)
Born18 September 1819
Died11 February 1868(1868-02-11) (aged 48)
Residence France
Nationality French
Alma mater University of Paris
Known for Foucault pendulum, eddy currents
Awards Copley Medal (1855)
Scientific career
Fields Physics
Institutions Paris Observatory

Jean Bernard Léon Foucault (French:  [ʒɑ̃ bɛʁnaʁ leɔ̃ fuko] ; 18 September 1819 – 11 February 1868) was a French physicist best known for his demonstration of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation. He also made an early measurement of the speed of light, discovered eddy currents, and is credited with naming the gyroscope.

Physicist scientist who does research in physics

A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate causes of phenomena, and usually frame their understanding in mathematical terms. Physicists work across a wide range of research fields, spanning all length scales: from sub-atomic and particle physics, through biological physics, to cosmological length scales encompassing the universe as a whole. The field generally includes two types of physicists: experimental physicists who specialize in the observation of physical phenomena and the analysis of experiments, and theoretical physicists who specialize in mathematical modeling of physical systems to rationalize, explain and predict natural phenomena. Physicists can apply their knowledge towards solving practical problems or to developing new technologies.

Foucault pendulum simple device conceived as an experiment to demonstrate the rotation of the Earth

The Foucault pendulum or Foucault's pendulum is a simple device named after French physicist Léon Foucault and conceived as an experiment to demonstrate the Earth's rotation. The pendulum was introduced in 1851 and was the first experiment to give simple, direct evidence of the earth's rotation. Today, Foucault pendulums are popular displays in science museums and universities.

Speed of light speed at which all massless particles and associated fields travel in vacuum

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second. It is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second. According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc2.


Early years

Foucault was the son of a publisher in Paris, where he was born on 18 September 1819. After an education received chiefly at home, he studied medicine, which he abandoned in favour of physics due to a blood phobia. [1] He first directed his attention to the improvement of Louis Daguerre's photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic anatomy.

Paris Capital of France

Paris is the capital and most populous city of France, with an area of 105 square kilometres and an official estimated population of 2,140,526 residents as of 1 January 2019. Since the 17th century, Paris has been one of Europe's major centres of finance, diplomacy, commerce, fashion, science, and the arts.

Physics Study of the fundamental properties of matter and energy

Physics is the natural science that studies matter, its motion, and behavior through space and time, and that studies the related entities of energy and force. Physics is one of the most fundamental scientific disciplines, and its main goal is to understand how the universe behaves.

Blood phobia is extreme and irrational fear of blood, a type of specific phobia. Severe cases of this fear can cause physical reactions that are uncommon in most other fears, specifically vasovagal syncope (fainting). Similar reactions can also occur with trypanophobia and traumatophobia. For this reason, these phobias are categorized as "blood-injection-injury phobia" by the DSM-IV. Some early texts refer to this category as "blood-injury-illness phobia."

With Hippolyte 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.

Hippolyte Fizeau French physicist

Armand Hippolyte Louis Fizeau FRS FRSE MIF was a French physicist, best known for measuring the speed of light in the namesake Fizeau experiment.

Light electromagnetic radiation in or near visible spectrum

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of roughly 430–750 terahertz (THz).

Sun Star at the centre of the Solar System

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

In 1849, Foucault experimentally demonstrated that absorption and emission lines appearing at the same wavelength are both due to the same material, with the difference between the two originating from the temperature of the light source. [2] [3]

Middle years

Foucault's pendulum in the Panthéon, Paris

In 1850, he did 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 corpuscular theory of light when it showed that light travels more slowly through water than through air. [4] In 1851, he provided an experimental demonstration of the rotation of the Earth on its axis (diurnal motion). This experimental setup had been used by Vincenzo Viviani but became well known to the public by Foucault's work. Foucault achieved the demonstration by showing the rotation of the plane of oscillation of a long and heavy pendulum suspended from the roof of the Panthéon, Paris. The experiment caused a sensation in both the learned and popular worlds, and " Foucault pendulums " were suspended in major cities across Europe and America and attracted crowds. In the following year he used (and named) the gyroscope as a conceptually simpler experimental proof. In 1855, he received the Copley Medal of the Royal Society for his 'very remarkable experimental researches'. Earlier in the same year he was made physicien (physicist) at the imperial observatory at Paris.

The Fizeau–Foucault apparatus is either of two types of instrument historically used to measure the speed of light. The conflation of the two instrument types arises in part because Hippolyte Fizeau and Léon Foucault had originally been friends and collaborators. They worked together on such projects as using the Daguerreotype process to take images of the Sun between 1843 and 1845 and characterizing absorption bands in the infrared spectrum of sunlight in 1847.

Isaac Newton Influential British physicist and mathematician

Sir Isaac Newton was an English mathematician, physicist, astronomer, theologian, and author who is widely recognised as one of the most influential scientists of all time, and a key figure in the scientific revolution. His book Philosophiæ Naturalis Principia Mathematica, first published in 1687, laid the foundations of classical mechanics. Newton also made seminal contributions to optics, and shares credit with Gottfried Wilhelm Leibniz for developing the infinitesimal calculus.

In optics, the corpuscular theory of light, arguably set forward by Descartes (1637) states that light is made up of small discrete particles called "corpuscles" which travel in a straight line with a finite velocity and possess impetus. This was based on an alternate description of atomism of the time period. This theory cannot explain refraction, diffraction and interference.

In September 1855 he 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.

Force Any action that tends to maintain or alter the motion of an object

In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity, i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F.

Copper Chemical element with atomic number 29

Copper is a chemical element with symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

Magnet material or object that produces a magnetic field

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.

Diagram of a variant of Foucault's speed of light experiment where a modern laser is the source of light Speed of light (foucault).PNG
Diagram of a variant of Foucault's speed of light experiment where a modern laser is the source of light

In 1857 Foucault invented the polarizer which bears his name, [5] and in the succeeding year devised a method of testing the mirror of a reflecting telescope to determine its shape. [6] [7] The so-called "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. In the test, light from a point source is focused onto the center of curvature of the mirror 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 in use to this date, most notably by amateur and smaller commercial telescope makers as it is inexpensive and uses simple, easily made equipment.

With Charles Wheatstone’s revolving mirror he, in 1862, determined the speed of light to be 298,000 km/s – 10,000 km/s less than that obtained by previous experimenters and only 0.6% in error of the currently accepted value.

Later years

In 1862 Foucault was made a member of the Bureau des Longitudes and an officer of the Legion of Honour. He became a member of the Royal Society of London in 1864, and member of the mechanical section of the Institute a year later. In 1865 he published his papers on a modification of James Watt's centrifugal governor; he had for some time been experimenting with a view to making its period of revolution constant and developing a new apparatus for regulating the electric light. Foucault showed how, by the deposition of a transparently thin film of silver on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye. His chief scientific papers are to be found in the Comptes Rendus, 1847–1869. Near his death he returned to Roman Catholicism that he previously abandoned. [8]

Death and afterwards

Grave of Jean Bernard Leon Foucault in Montmartre Cemetery Foucault jean bernard leon grave montmartre paris 01.jpg
Grave of Jean Bernard Léon Foucault in Montmartre Cemetery

Foucault died of what was probably a rapidly developing case of multiple sclerosis [9] on 11 February 1868 in Paris and was buried in the Montmartre Cemetery.

The asteroid 5668 Foucault was named for him. [10] His is one of the 72 names inscribed on the Eiffel Tower.


Collected Works:

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  1. PD-icon.svg Herbermann, Charles, ed. (1913). "Jean-Bertrand-Léon Foucault". Catholic Encyclopedia . New York: Robert Appleton Company.
  2. Brand, John C. D. (1995). Lines of Light: The Sources of Dispersive Spectroscopy. Luxembourg: Gordon and Breach Publishers. pp. 60–62. ISBN   978-2884491624.
  3. See:
    • Foucault, L. (7 February 1849). "Lumière électrique" [Electric light]. l'Institut, Journal Universel des Sciences … (in French). 17 (788): 44–46.
    • Foucault, L. (1849). "Lumière électrique" [Electric light]. Société Philomatique de Paris. Extraits des Procès-Verbaux de Séances. (in French): 16–20.
  4. David Cassidy; Gerald Holton; James Rutherford (2002). Understanding Physics. Birkhäuser. ISBN   0-387-98756-8.
  5. Léon Foucault (August 17, 1857) "Nouveau polariseur en spath d'Island. Expérience de fluorescence" (New polarizer made of Icelandic spar. Fluorescence experiment.), Comptes rendus, vol. 45, pages 238–241. English translation: Léon Foucault (1857) "On a new polarizer of Iceland spar. Experiment on fluorescence.," The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, series 4, vol. 14, pages 552–555.
  6. L. Foucault (1858) "Description des procédés employes pour reconnaitre la configuration des surfaces optiques" (Description of the methods used to recognize the configuration of optical surfaces), Comptes rendus ... , vol. 47, pages 958–959.
  7. L. Foucault (1859) "Mémoire sur la construction des télescopes en verre argenté" (Memoir on the construction of reflecting telescopes), Annales de l'Observatoire impériale de Paris, vol. 5, pages 197–237.
  8. William Tobin (2003). The Life and Science of Léon Foucault: The Man Who Proved the Earth Rotates. Cambridge University Press. p. 272. ISBN   9780521808552.
  9. W. Tobin, The Life and Science of Léon Foucault, Cambridge University Press (2003).
  10. Schmadel, Lutz D.; International Astronomical Union (2003). Dictionary of minor planet names. Berlin; New York: Springer-Verlag. p. 480. ISBN   978-3-540-00238-3 . Retrieved 9 September 2011.

Further reading

External video
Nuvola apps kaboodle.svg Presentation by Aczel on Pendulum: Léon Foucault and the Triumph of Science, September 23, 2003, C-SPAN