Augustin-Jean Fresnel was a French engineer and physicist. He was in the employment of the Corps of Bridges, Waters and Forests for most part of his life.
Background
Augustin-Jean Fresnel was born on May 10, 1788, at Broglie, France. His father, Jacques, was a successful Norman architect and building contractor. In 1785, while directing improvements on the château of the maréchal de Broglie, he married Augustine Mérimée, the pious, well-educated daughter of the estate’s overseer. Subsequently he was employed on the harbor construction project at Cherbourg; and when this work was interrupted by the Revolution in 1794, he retired with his family to Mathieu, north of Caen. Here Augustin spent the remainder of his childhood, deeply influenced by the home.
Education
Fresnel was initially home-schooled by his mother. He was considered the slow one, hardly beginning to read until the age of eight. At nine and ten he was undistinguished except for his ability to turn tree-branches into toy bows and guns that worked far too well, earning himself the title l'homme de génie (the man of genius) from his accomplices, and a united crackdown from their elders. At twelve he entered the École Centrale in Caen. Intending a career in engineering, Fresnel was admitted to the École Polytechnique in Paris in 1804. Graduating in 1806, he then enrolled at the École Nationale des Ponts et Chaussées, from which he graduated in 1809.
Career
After completing his formal training, Augustin entered government service as a civil engineer. The Corps des Ponts et Chaussées first assigned him to Vendée, where he worked on the roads linking the department with its new chef-lieu at La Roche-sur-Yon. About 1812 he was sent to Nyon, France, to assist with the imperial highway which was to connect Spain with Italy through the Alpine pass at Col Montgenèvre. In moments snatched from his professional duties Fresnel diverted himself with a series of philosophical, technical, and scientific concerns.
Fresnel began his research in optics in 1814. He lost his post temporarily during the period following Napoleon’s return from Elba in 1815. At the beginning of the 19th century, the scientific community championed Isaac Newton’s corpuscular, or particle, theory of light. However, in 1802 Young showed that an interference pattern is produced when light from two sources overlaps, which could happen only if light was a wave. Fresnel initially did not know about Young’s experiment, but his experiments with various devices for producing interference fringes and diffraction convinced him that the wave theory of light was correct. As a starting point for his mathematical description of diffraction, Fresnel used Dutch scientist Christiaan Huygens’s principle that every point on a wave front can be considered a secondary source of spherical wavelets.
Fresnel presented his work on diffraction as an entry to a competition on the subject sponsored by the French Academy of Sciences in 1819. The committee of judges included a number of prominent advocates of Newton’s corpuscular model of light, one of whom, mathematician Siméon-Denis Poisson, pointed out that Fresnel’s model predicted a seemingly absurd result: if a parallel beam of light falls on a small spherical obstacle, there will be a bright spot at the centre of the circular shadow - a spot nearly as bright as if the obstacle was not there at all. An experiment was subsequently performed by the French physicist François Arago, and the spot (subsequently called Poisson’s spot) was seen, vindicating Fresnel, who won the competition.
Despite this triumph for the wave theory of light, the properties of polarized light could seemingly be explained only by the corpuscular theory, and beginning in 1816 Fresnel and Arago studied the laws of the interference of polarized light. In 1817 he was the first to obtain circularly polarized light. This discovery led him to the conclusion that light was not a longitudinal wave as previously supposed but a transverse wave. Young had independently reached the same conclusion.
On the recommendation of Arago, in 1819 Fresnel joined Arago on a government committee to improve French lighthouses. In 1821 he produced his first apparatus using the refracting properties of glass, now known as the dioptric system. On a lens panel he surrounded a central bull’s-eye lens with a series of concentric glass prismatic rings. The panel collected light emitted by the lamp over a wide horizontal angle and also the light that would otherwise escape to the sky or to the sea, concentrating it into a narrow horizontal pencil beam. With a number of lens panels rotating around the lamp, Fresnel was then able in 1824 to produce several revolving beams from a single light source, an improvement over the mirror that produces only a single beam. To collect more of the light wasted vertically, he added above and below the main lens triangular prism sections that both refracted and reflected the light. By doing this he considerably steepened the angle of incidence at which rays shining up and down could be collected and made to emerge horizontally. Thus emerged the full Fresnel catadioptric system.
Although his work in optics received scant public recognition during his lifetime, Fresnel maintained that not even acclaim from distinguished colleagues could compare with the pleasure of discovering a theoretical truth or confirming a calculation experimentally.
Religion
Augustin regarded his intellectual talents as gifts from God, and considered it his duty to use them for the benefit of others. Plagued by poor health, and determined to do his duty before death thwarted him, he shunned pleasures and worked to the point of exhaustion.
Membership
Fresnel was a foreign member of the Royal Society of London.
Personality
Despite the urgency of everything he attempted, Fresnel was always attentive to detail, systematic, and thorough. In science no less than in politics he held tenaciously to his convictions and defended them with courage and vigor. As a functionary he voiced outrage when the behavior of others fell short of his own high ethical standards. At times this approached a rankling self-righteousness, but generally his contemporaries saw him as reserved, gentle, and charitable.