Augustin-Jean Fresnel
Augustin-Jean Fresnel was a French civil engineer and physicist whose research in optics led to the almost unanimous acceptance of the wave theory of light, fully supplanting Newton's corpuscular theory, from the late 1830s until the end of the 19th century. He is perhaps better known for inventing the catadioptric Fresnel lens and for pioneering the use of "stepped" lenses to extend the visibility of lighthouses, saving countless lives at sea. The simpler dioptric stepped lens, first proposed by Count Buffon and independently reinvented by Fresnel, is used in screen magnifiers and in condenser lenses for overhead projectors.
Fresnel gave the first satisfactory explanation of diffraction by straight edges, including the first satisfactory wave-based explanation of rectilinear propagation. By further supposing that light waves are purely transverse, Fresnel explained the nature of polarization. He then worked on double refraction.
Fresnel had a lifelong battle with tuberculosis, to which he succumbed at the age of 39. He lived just long enough to receive recognition from his peers, including the Rumford Medal of the Royal Society, and his name is ubiquitous in the modern terminology of optics and waves. After the wave theory of light was subsumed by Maxwell's electromagnetic theory in the 1860s, some attention was diverted from the magnitude of Fresnel's contribution. In the period between Fresnel's unification of physical optics and Maxwell's wider unification, a contemporary authority, Humphrey Lloyd, described Fresnel's transverse-wave theory as "the noblest fabric which has ever adorned the domain of physical science, Newton's system of the universe alone excepted".
Early life
Family
Augustin-Jean Fresnel, born in Broglie, Normandy, on 10 May 1788, was the second of four sons of the architect Jacques Fresnel and his wife Augustine, née Mérimée. The family moved twice—in 1789/90 to Cherbourg, and in 1794 to Jacques's home town of Mathieu, where Augustine would spend 25 years as a widow.The first son, Louis, was admitted to the École Polytechnique, became a lieutenant in the artillery, and was killed in action at Jaca, Spain. The third, Léonor, followed Augustin into civil engineering, succeeded him as secretary of the Lighthouse Commission, and helped to edit his collected works. The fourth, Fulgence Fresnel, became a linguist, diplomat, and orientalist, and occasionally assisted Augustin with negotiations. Fulgence died in Baghdad in 1855 having led a mission to explore Babylon.
Madame Fresnel's younger brother, Jean François "Léonor" Mérimée, father of the writer Prosper Mérimée, was a painter who turned his attention to the chemistry of painting. He became the Permanent Secretary of the École des Beaux-Arts and a professor at the École Polytechnique.
Education
The Fresnel brothers were initially home-schooled by their mother. The sickly Augustin was considered the slow one, not inclined to memorization; but the popular story that he hardly began to read until the age of eight is disputed. At the age of nine or 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 from his accomplices, and a united crackdown from their elders.In 1801, Augustin was sent to the École Centrale at Caen, as company for Louis. But Augustin lifted his performance: in late 1804 he was accepted into the École Polytechnique, being placed 17th in the entrance examination. As the detailed records of the École Polytechnique begin in 1808, we know little of Augustin's time there, except that he made few if any friends and—in spite of continuing poor health—excelled in drawing and geometry: in his first year he took a prize for his solution to a geometry problem posed by Adrien-Marie Legendre. Graduating in 1806, he then enrolled at the École Nationale des Ponts et Chaussées, from which he graduated in 1809, entering the service of the Corps des Ponts et Chaussées as an ingénieur ordinaire aspirant. Directly or indirectly, he was to remain in the employment of the "Corps des Ponts" for the rest of his life.
Religious formation
Fresnel's parents were Roman Catholics of the Jansenist sect, characterized by an extreme Augustinian view of original sin. Religion took first place in the boys' home-schooling. In 1802, his mother said:Augustin remained a Jansenist. He regarded his intellectual talents as gifts from God, and considered it his duty to use them for the benefit of others. According to his fellow engineer Alphonse Duleau, who helped to nurse him through his final illness, Fresnel saw the study of nature as part of the study of the power and goodness of God. He placed virtue above science and genius. In his last days he prayed for "strength of soul", not against death alone, but against "the interruption of discoveries ... of which he hoped to derive useful applications".
Jansenism is considered heretical by the Roman Catholic Church, and Grattan-Guinness suggests this is why Fresnel never gained a permanent academic teaching post; his only teaching appointment was at the Athénée in the winter of 1819–20. The article on Fresnel in the Catholic Encyclopedia does not mention his Jansenism, but describes him as "a deeply religious man and remarkable for his keen sense of duty".
Engineering assignments
Fresnel was initially posted to the western of Vendée. There, in 1811, he anticipated what became known as the Solvay process for producing soda ash, except that recycling of the ammonia was not considered. That difference may explain why leading chemists, who learned of his discovery through his uncle Léonor, eventually thought it uneconomic.About 1812, Fresnel was sent to Nyons, in the southern of Drôme, to assist with the imperial highway that was to connect Spain and Italy. It is from Nyons that we have the first evidence of his interest in optics. On 15 May 1814, while work was slack due to Napoleon's defeat, Fresnel wrote a postscript to his brother Léonor, saying in part:
As late as 28 December he was still waiting for information, but by 10 February 1815 he had received Biot's memoir.
In March 1815, perceiving Napoleon's return from Elba as "an attack on civilization", Fresnel departed without leave, hastened to Toulouse and offered his services to the royalist resistance, but soon found himself on the sick list. Returning to Nyons in defeat, he was threatened and had his windows broken. During the Hundred Days he was placed on suspension, which he was eventually allowed to spend at his mother's house in Mathieu. There he used his enforced leisure to begin his optical experiments.
Contributions to physical optics
Fresnel made major contributions to several areas of physical optics. These included studies of diffraction, where he explained the colored fringes seen in shadows of objects illuminated by narrow beams, and conducted double-mirror experiments. He studied polarization, discovering that the two images produced by a birefringent crystal could not be combined to create a diffraction pattern. A third area that he studied was double refraction, where he found that neither of the two refractions in a topaz crystal could have been produced by ordinary spherical secondary waves.Lighthouses and the Fresnel lens
On 21 June 1819, Fresnel was "temporarily" seconded by the Commission des Phares to review possible improvements in lighthouse illumination.By the end of August 1819, Fresnel recommended lentilles à échelons to replace the reflectors then in use, which reflected only about half of the incident light. Where Buffon's version was biconvex and in one piece, Fresnel's was plano-convex and made of multiple prisms for easier construction. In a public spectacle on the evening of 13 April 1821, his design was demonstrated by comparison with the most recent reflectors, which it suddenly rendered obsolete.
Fresnel's next lens was a rotating apparatus with eight "bull's-eye" panels, made in annular arcs by Saint-Gobain, giving eight rotating beams—to be seen by mariners as a periodic flash. Above and behind each main panel was a smaller, sloping bull's-eye panel of trapezoidal outline with trapezoidal elements. The official test, conducted on the unfinished Arc de Triomphe on 20 August 1822, was witnessed by the commission—and by Louis XVIII and his entourage—from 32km away. The apparatus was reassembled at Cordouan Lighthouse under Fresnel's supervision. On 25 July 1823, the world's first lighthouse Fresnel lens was lit.
In May 1824, Fresnel was promoted to secretary of the Commission des Phares, becoming the first member of that body to draw a salary, albeit in the concurrent role of Engineer-in-Chief.
In the same year he designed the first fixed lens—for spreading light evenly around the horizon while minimizing waste above or below, in a beehive-shaped design. The second Fresnel lens to enter service was a fixed lens, of third order, installed at Dunkirk by 1 February 1825. It had a 16-sided polygonal plan.
In 1825, Fresnel extended his fixed-lens design by adding a rotating array outside the fixed array. Each panel of the rotating array was to refract part of the fixed light from a horizontal fan into a narrow beam.
Also in 1825, Fresnel unveiled the Carte des Phares, calling for a system of 51 lighthouses plus smaller harbor lights, in a hierarchy of lens sizes, with different characteristics to facilitate recognition: a constant light, one flash per minute, and two per minute.
In late 1825, to reduce the loss of light in the reflecting elements, Fresnel proposed to replace each mirror with a catadioptric prism, through which the light would travel by refraction through the first surface, then total internal reflection off the second surface, then refraction through the third surface. The result was the lighthouse lens as we now know it. In 1826 he assembled a small model for use on the Canal Saint-Martin.