A Short Bright Flash: Augustin Fresnel and the Birth of the Modern Lighthouse

  • Theresa Levitt
W. W. Norton: 2013 9780393068795 | ISBN: 978-0-3930-6879-5

At a remote lighthouse on the English coast, I toured the tower and saw a starfish of beams reaching 20 kilometres across the sea from its revolving glass crown. I imagined the light sucking vast amounts of power from the electric grid, until I saw the bulb: a 20-watt halogen candle smaller than my thumb. It is not in the bulb, but in the glass encasing it, that the power lies.

A Fresnel lens inside Cape Blanco Lighthouse in Oregon. Credit: QUANG-TUAN LUONG/TERRAGALLERIA.COM

Nineteenth-century French physicist Augustin Fresnel worked the physical magic of this lens. The light is focused into beams by concentric rings of prisms, each an inch thick and stacked in metre-high panes set into a brass frame. As the apparatus turns on a bed of oil, the beam sweeps across the waves. In A Short Bright Flash, historian Theresa Levitt recounts how these lenses came to keep ships safe near the shore and how they spread around the world.

Much trade was conducted by sea in the early nineteenth century and hundreds of vessels ran aground in Europe each year, most at night. Newspapers were filled with stories of shipwrecks and lost crews. France had 20 lighthouses by 1800; Britain, a handful more. These relied on the dim lighting of oil lamps reflected by mirrors, a technology little changed since the first lighthouses of the ancient world — the third-century bc Colossus of Rhodes and Pharos of Alexandria.

Fresnel spent years trying to understand the diffraction of light, first in his spare time while building roads across France for Napoleon, then while jostling with scientific luminaries in Paris. Influenced by physicist and astronomer François Arago — famous for studies on light and magnetism, and helping to discover the planet Neptune — Fresnel came up with controversial extensions to the wave theory of light. In 1819, Arago invited Fresnel to work at the Lighthouse Commission in France, created in 1811 by Napoleon to spread 'beacons of light' around his empire. Fresnel saw that lenses would do a better job than mirrors.

The problem was how to capture enough light and focus it. A lens close to a lamp would intercept most of its glare, but forming a beam necessitates bending the rays through a large angle and would require a bulbous lens, which would absorb more of the light. To capture the same amount of light, a thinner lens farther away would have to be larger than could be manufactured at the time. At a demonstration to the commission in 1821, Fresnel placed his bull's-eye lens alongside other reflectors high on the Paris Observatory. The public and commissioners across the city saw that Fresnel's was by far the brightest.

Fresnel built a lens for the country's most prestigious lighthouse at Cordouan, the 'Versailles of the sea' at the mouth of the Gironde estuary near Bordeaux. Installed in July 1823, its beams could be seen by tall ships up to 33 nautical miles (61 kilometres) away. In 1825, Fresnel revealed a map for 51 lighthouses around the French coast of different sizes and illumination patterns. Two years later he was dead from tuberculosis, and it was left to his brother Leonor to complete the scheme in 1854.

Hundreds of lighthouses with Fresnel lenses had been built by then, from Africa to Brazil. The United States was an exception. Stephen Pleasonton, accountant of the US Lighthouse Establishment, notably refused to sanction the purchase of the lenses, citing cost. US lights burned sperm whale oil at the time and Pleasonton insisted that more efficient lamps were the way to economize.

But with westward expansion and the advent of railways and the telegraph, science and technology rose in the country's esteem. In 1852, Congress passed a bill to create the US Lighthouse Board after several congressmen were stuck in fog on a boat to New York and saw how bad existing lighthouses were. Fresnel lenses were then systematically introduced. Levitt describes how the disabling of lighthouses played a central part in both the American Civil War and the Second World War.

The invention of radar meant that few lighthouses were built and many were decommissioned after the Second World War. Yet working towers retain their Fresnel lenses, which also appear in everything from traffic lights to overhead projectors. Levitt's detailed history is worth ploughing through to see how important scientists and engineers have been in saving sailors' lives. The French saying 'Faire rayonner la France' ('Make France radiant') sums it up. Fresnel indeed lit up his country and the world.