Augustin Fresnel's stepped lens made lighthouses visible for miles
Suspended from his road-building job after opposing Napoleon's return from Elba, a sickly French engineer used the idle months at his mother's house to begin experimenting with light. Within a decade Augustin Fresnel had helped establish that light is a wave and had lit the world's first lighthouse lens.
Fresnel was born in Broglie, Normandy, on 10 May 1788, the second of four sons of an architect; his cousin was the writer Prosper Mérimée. Home-schooled in a strict Jansenist household, he was considered the slow brother, though at nine or ten he made toy bows and guns from branches that worked alarmingly well. He entered the École Polytechnique in 1804, won a prize for a geometry problem set by Adrien-Marie Legendre, and joined the state corps of bridges and roads in 1809, remaining in its service for life.
In 1814, working on an imperial highway in Nyons, he confessed to his brother that he could not guess what the polarization of light was. The enforced leisure of 1815 changed that. Between 1815 and 1818 he explained the coloured fringes at the edges of shadows and gave the first satisfying wave account of why light travels in straight lines. Proposing that light waves vibrate purely sideways, he then explained polarization, and went on to study double refraction in crystals such as topaz. His work displaced Newton's particle theory, and Humphrey Lloyd ranked it second only to Newton's system of the universe.
In 1819 France's lighthouse commission asked him to improve its lights. The reflectors then in use returned only about half the light, so he proposed lenses built in steps from multiple prisms, easier to make than the one-piece biconvex version the Count de Buffon had once suggested. A public demonstration on 13 April 1821 made the old reflectors obsolete overnight. A rotating eight-panel lens was tested from the unfinished Arc de Triomphe in 1822, watched by Louis XVIII from 32 kilometres away, and on 25 July 1823 it shone from Cordouan Lighthouse. He later planned a national network of 51 lighthouses with distinctive flash patterns.
Recognition came slowly. He won a single vote when he first applied to the Académie des Sciences, lost to Joseph Fourier, and was elected unanimously only in 1823. Tuberculosis dogged him throughout his life and killed him at 39 on 14 July 1827, shortly after the Royal Society's Rumford Medal reached him on his deathbed.
Source: Augustin-Jean Fresnel
