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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Augustin-Jean Fresnel | 2/5 | https://en.wikipedia.org/wiki/Augustin-Jean_Fresnel | reference | science, encyclopedia | 2026-05-05T16:29:08.023602+00:00 | kb-cron |
== Engineering assignments == Fresnel was initially posted to the western département 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 département 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:
I would also like to have papers that might tell me about the discoveries of French physicists on the polarization of light. I saw in the Moniteur of a few months ago that Biot had read to the Institute a very interesting memoir on the polarization of light. Though I break my head, I cannot guess what that is. As late as 28 December he was still waiting for information, but by 10 February 1815 he had received Biot's memoir. (The Institut de France had taken over the functions of the French Académie des Sciences and other académies in 1795. In 1816 the Académie des Sciences regained its name and autonomy, but remained part of the institute.) 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 (1815–1818), where he explained the colored fringes seen in shadows of objects illuminated by narrow beams, and conducted double-mirror experiments. He studied polarization (1816–1823), 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 (1821–1826), 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 (Commission of Lighthouses) to review possible improvements in lighthouse illumination. By the end of August 1819, Fresnel recommended lentilles à échelons (lenses by steps) 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 32 km 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 (Lighthouse Map), calling for a system of 51 lighthouses plus smaller harbor lights, in a hierarchy of lens sizes (called orders, the first order being the largest), with different characteristics to facilitate recognition: a constant light (from a fixed lens), one flash per minute (from a rotating lens with eight panels), and two per minute (sixteen panels).
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.
== Honors ==