Thursday, June 05, 2014

Podcast: Fresnel and the Lighthouse


The painting you see here is called The Raft of the Medusa. It was completed in 1819 by the French artist Theodore Gericault, and it's based on a true story. The ship the Medusa ran aground off the coast of Africa in 1816 and more than 100 people found themselves stranded on a makeshift raft. 15 people made it out alive, after surviving on the raft for nearly two weeks. Later, the survivors would tell one of the most grisly tales of shipwreck the world had ever heard. The first night on the raft people were swept away by bad weather, or crushed between the raft's planks. There were no supplies on the ship, save a few casks of wine, so the cannibalism started quickly. At one point, the stronger half of the survivors thought the weaker half were going to die anyway, so they decided to murder them.

The world watched with fascination and horror as the story unfolded, and at least one person thought to himself: "This has got to stop."* So the physicist Augustin Fresnel, who had fought his way from obscurity to a position as one of France's leading theoretical physicists, committed himself to building better lighthouses.



Today, most people imagine lighthouses as having those bright, brilliant beams that can be seen all the way to the horizon. Truthfully, before the 1820's, lighthouses could only be seen for about 5 or 6 miles, which was far enough to guide ships into harbors, but not nearly far enough to steer them away from dangerous shores. Because of this, shipwrecks caused by collisions with land were alarmingly frequent. It was estimated that one hundred ships per year went down in the English Chanel alone.

With the power of physics, Fresnel thought he could make lighthouses bright enough to warn ships away from shoals and rocky outcroppings. To do this, he would use a convex lens (like a magnifying glass) to focus the light from a fire or a lamp down into a single beam. But because building a magnifying glass large enough to fit in a lighthouse was impossible at the time (and remains totally impractical) he had to adapt the lens to fit the purpose.

This is how the Fresnel lens was born.

If there needs to be a demonstration of just how powerful the Fresnel lens was, note that during the American Civil War, lighthouses (which had just recently been equipped with the new Fresnel lenses) became a major strategic holding point. The Confederacy, determined to sabotage the Union blockade, shut down all the lighthouses along the coast. It hid the Fresnel lenses or destroyed them. The Union made a priority to relight the coast, which it pursued through the war's end.

Listen to the podcast to hear more of the story and the physics behind Fresnel lenses, featuring science historian Theresa Levitt, author of A Short Bright Flash: Augustin Fresnel and the Birth of the Modern Lighthouse. 

For more info about convex and concave lenses, or specifically about Fresnel lenses, check the links.

*Not a direct quote. The wreck of the Medusa was one of many shipwrecks at that time and may or may not have directly inspired Fresnel's work on lenses. It did take place at about the same time that Fresnel was making a name for himself in the elite physics community in France.

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