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No. 2867:
Fresnel and Lighthouses

Today, Fresnel and his Lighthouses. The University of Houston's College of Engineering presents this series about the machines that make our civilization run and about the people whose ingenuity created them. 

I was struck, on a visit to Key West, by a string of lighthouses: One in the town itself; another on Fort Jefferson, seventy miles away in the Dry Tortugas. Three miles further, rose the huge Loggerhead Key light. I didn't grasp the full significance of all those lights until I read Theresa Levitt's remarkable book: A Short Bright Flash. It's part biography of Augustin Fresnel, part history of our US lighthouse system. 

Key West LightFort Jefferson LightDry Tortugas Light
Left to right: Key West Lighthouse, Fort Jefferson Lighthouse, Dry Tortugas Lighthouse

Augustin FresnelFirst, Fresnel: He was born in Normandy in 1788. He studied math and bridge-building; then worked as a road engineer. But his heart lay with the theory of light. He spent the next decade demonstrating that light had wavelike properties. 

That flew in the teeth of Newton's corpuscular view of light and it grievously offended the reigning voice of French physics, Simon LaPlace. But, with support from Fran�ois Arago, Fresnel prevailed. He, more than anyone, solidified the wave/particle contradiction. And it wouldn't be resolved 'til we had quantum mechanics.

Here the story takes an odd twist: Napoleon had set up a Lighthouse Commission to create better shore warnings for French shipping. Fresnel was called to serve on it. The result was a wondrous convergence of science and public service.

How Fresnel modified a spherical lensLighthouses were then using oil-fed flames in front of roughly parabolic reflectors. Fresnel realized how much light was lost in that system. A light inside a spherical lens would be far brighter, but the lens would have to be huge. He got around that by stacking horizontal slices of a spherical lens in one plane. His lens had several such panels and slowly rotated, each panel signaling to sailors with -- a short bright flash.

It took years to solve all the problems of making such lenses. They were still improving when Fresnel, still young, died of tuberculosis. By then his lights girdled France, and they'd soon girdle Europe.

Here in America, our need for far more, far better, lighthouses was urgent. Example: The shoals off North Carolina lay ten miles beyond the Hatteras light. But on most nights those pale reflector lights didn't reach ten miles. Countless ships perished as they steered toward shore looking for that light.

Bureaucrat Stephen Pleasonton, who long oversaw our lighthouses, stubbornly opposed the newfangled French technology -- fought every attempt to adopt it. Not until shortly before the Civil War did we finally have the lights our coastline so badly needed. Then, during the Union Blockade, the South blacked out its own lights, and attacked those controlled by the North.

Key West lens
1858 first-order Fresnel lens made by Augustin Henry Lepaute of Paris. It served in yet another Florida Keys lighthouse, the Sombrero Key light

After that we depended on our coastal Fresnel lights for over a century. Now electronics warn our ships, and lighthouses are mere grist for tourists. We puzzle over those huge multi-faceted glass lenses -- green and glorious. They could be modern art sculptures. Yet they tell of a brilliant high-technology that reshaped our history.

I'm John Lienhard at the University of Houston, where we�re interested in the way inventive minds work.

(Theme music)

I strongly recommend the book: T. Levitt, A Short Bright Flash. (New York: W.W. Norton and Co., 2013)

But see also the Wikipedia entries on the relevant topics, e.g.: Augustin-Jean FresnelFresnel lensThe Huygens-Fresnel principleThe Arago SpotStephen Pleasonton, and this page about Fresnel lenses.

Fresnel's supporter François Arago is a story all by himself. See e.g., Engines episodes 691and 704. :

Images for the web version: All color photos by J. H. Lienhard, B&W images courtesy Wikimedia Commons.

A fourth-order Fresnel lens on display at the Maine Lighthouse Museum

This episode was first aired on March 6, 2013