Episode notes
In the early 19th century most lighthouses were stone towers with coal fires or weak oil lamps, and Thomas Rogers's thick glass mirrors of the 1780s lost much of their light to bubbles and impurities. Napoleon created the French Commission of Lighthouses in 1811, and in 1819 it brought in Augustin-Jean Fresnel, an engineer with the Corps of Bridges and Roads who had just won a French Academy of Sciences prize for work on diffraction. He proposed a stepped lens that kept only the refracting surfaces of a convex lens, collapsed into concentric prisms, and was told the Comte de Buffon had suggested stepped lenses in 1748, though Buffon imagined a single heavy burning glass. With a 500 franc budget, glassmaker François Soleil, and François Arago's multi-wick lamp, he delivered a 97-prism prototype in March 1820, its prisms held with isinglass, a glue made from sturgeon swim bladders.
In 1822 a rotating array of eight panels atop the unfinished Arc de Triomphe was seen from 20 miles away by Louis XVIII and the commission, and in July 1823 Fresnel installed a lens in the Cordouan lighthouse at the mouth of the Gironde. In the same years he was showing that light travels as transverse waves, overturning Newton's particle theory. After developing tuberculosis, he left the École Polytechnique and his theoretical research in 1824 to finish the lighthouse program, and his 1825 lighthouse map planned 51 lights. He died on July 14, 1827, at 39, and his brother Léonor carried on the rollout.
- His lenses came in six orders, from first-order giants over eight feet tall to sixth-order harbor lights.
- Rotating lenses gave each lighthouse a flash signature sailors could time to fix their position.
- His fully catadioptric design used total internal reflection in glass to replace metal mirrors.
- Alan and Thomas Stevenson brought his lenses to Britain, and many in the American South were buried or smashed in the Civil War.
- Fresnel lenses now work in theater spotlights, carrier landing systems, solar concentrators, and VR headsets.