On February 26, 1786, French mathematician, physicist, and astronomer François Arago was born. Arago discovered the principle of the production of magnetism by rotation of a nonmagnetic conductor. He also devised an experiment that proved the wave theory of light and engaged with others in research that led to the discovery of the laws of light polarization.
Francois Arago – Early Years
Dominique-François-Jean Arago was born in Estagel, Roussillon, France. His father was the small town’s mayor and his family was involved in politics with mainly leftist republican views and military connections. He was educated at the Municipal College of Perpignan where he became interested in mathematics. Later, he was admitted to the École Polytechnique in Paris. There, he he succeeded Gaspard Monge in the chair of analytic geometry at the age of only 23. Subsequently, Arago also was director of the Paris Observatory and permanent secretary of the Academy of Sciences. He was also active as a republican in French politics. As minister of war and marine in the provisional government formed after the Revolution of 1848, he introduced many reforms.
Around 1820, Arago demonstrated that the passage of an electric current through a cylindrical spiral of copper wire caused it to attract iron filings as if it were a magnet and that the filings fell off when the current ceased. This work is based on the scientific research by H.C. Ørsted of Denmark . Shortly after, Aragon managed to show that a rotating copper disk produced rotation in a magnetic needle suspended above it. It was later Michael Faraday who proved these to be induction phenomena.
The Stranger Properties of Light
Alexis Thérèse Petit (1791-1820) married a sister of Aragos and the two of them jointly carried out experiments on the refraction of light and the influence of temperature on the refractive index in gases. In 1816, together with the latter, he found the law according to which two rays polarised in a plane can interfere with each other, but rays polarised perpendicular to each other cannot. François Arago is known to have been a great supporter of Augustin-Jean Fresnel’s wave theory of light. The theory states that light should be retarded as it passes from a rarer to a denser medium. Laplace, Biot, and Poisson however favorited the emission theory, which means that light should rather be accelerated. Before Arago’s death, the retardation of light in denser media was demonstrated by Fizeau and Léon Foucault, who used Arago’s method with improvements in detail.
A New Planet
From 1830 Arago was director of the Paris Observatory, where he used all modern possibilities of astronomy and physics. He was the first to attribute the scintillation of stars to interference in the Earth’s atmosphere caused by air turbulence. Arago took part in the dispute between Urbain Le Verrier, who was his protégé, and the English astronomer John C. Adams over priority in discovering the planet Neptune and over the naming of the planet. In 1845, Arago suggested that Le Verrier investigate anomalies in the motion of Uranus. When the investigation resulted in Le Verrier’s discovery of Neptune, Arago proposed that the newly found planet be named for Le Verrier.
An Advocat for Photography
On August 19, 1839, Arago officially presented the invention of photography by Louis Daguerre and Joseph Nicéphore Nièpce to the French Academy of Science and the public.[11, 12] Earlier, the Chamber of Deputies and the House of Pairs had approved a law whereby the rights to the invention were purchased by the French state and made available to the world as a gift. Daguerre received a life-long monthly pension of 6,000 francs and Isidore Nièpce, the son of Joseph Nicéphore Nièpce, received such a pension of 4,000 francs.
Arago remained a consistent republican to the end, and after the coup d’état of 1852, though suffering first from diabetes, then from Bright’s disease, complicated by dropsy, he resigned his post as astronomer rather than take the oath of allegiance. Napoleon III gave directions that the old man should be in no way disturbed, and should be left free to say and do what he liked. In the summer of 1853 Arago was advised by his physicians to try the effect of his native air, and he accordingly set out to the eastern Pyrenees, but this was ineffective and he died in Paris.
References and Further Reading:
-  Detailed Biography of Francois Arago at MacTutor History
-  Francois Arago at Britannica
-  Caspar Monge and the Geometry, SciHi Blog, May 10, 2014.
-  A Life of Discoveries – the great Michael Faraday, SciHi Blog, September 22, 2012.
-  Hippolyte Fizeau and the Speed of Light, SciHi Blog, September 23, 2014.
-  Fizeau, Foucault and Astronomical Photography, SciHi Blog, April 2, 2014.
-  Neptune, Oceanos, or ‘Le Verrier’ – How to name a new planet?, SciHi Blog, June 9, 2012.
-  Francois Arago at Wikidata
-  Hans Christian Ørsted connecting Electricity and Magnetism, SciHi Blog
-  Augustin-Jean Fresnel and the Wave Theory of Light, SciHi Blog
-  Making Photography Really Operational – Louis Daguerre, SciHi Blog
-  Nicéphore Niépce and the World’s First Photograph, SciHi Blog
-  Timeline for Francois Arago, via Wikidata