Science Makes a Better Lighthouse Lens

Science Makes a Better Lighthouse Lens

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Science Makes a Better Lighthouse Lens

State of the art, one of the seven wonders of the Ancient World, the great lighthouse of Alexandria, built around 280 B.C., towered some 450 feet above Egypt's greatest harbor. At that height, it was the second tallest structure in the world, after another of the seven — the Great Pyramid of Giza. The light within, also state of the art, was an open flame.

From that time until the 18th century, the lights that warned ships that they were approaching land improved hardly at all. Some burned coal. Others stuck with wood. Oil lamps backed by mirrors eventually offered a bit more candlepower. Still, every coastline in the world remained littered with the ribs of broken ships whose captains didn't see the lighthouse until it was too late. Then, in 1822, a frail scientist with a passion for optics made a revolutionary breakthrough. His name was Augustin Jean Fresnel.

An example of his greatest creation, a large object that looks like a crystalline beehive, may be seen inside the Hall of American Maritime Enterprise at the National Museum of American History. It towers above the museum's displays of ship models and marine artifacts. This is the Fresnel lens, not one lens really, but numerous tiers of prisms. Lenses like this one turned simple flames into beams upon which sea captains could take their bearings, avoiding risk to their ships and to the lives of their passengers.

As big as it seems, by lighthouse standards the Smithsonian glass beehive is just above average, measuring nearly six feet high and more than three feet wide. It was installed in the famous Bolivar Lighthouse overlooking Galveston, Texas, sometime around 1907. This lighthouse served as sentry, and for some as sanctuary, during the deadliest hurricane in American history. The lens is visible proof that unlike the sea, light can be mastered by human ingenuity.

 For thousands of years, lighthouses created their lights with open flames which, naturally, were ineffective during high winds or rain. Then, in the 1690s, the glass lantern room was invented and first installed in England's Eddystone Lighthouse. Candles placed in the lantern room's glass lantern burned more brightly and securely than the earlier open fires. Another improvement came when pieces of mirrors placed in huge, round, wooden bowls served as crude reflectors that helped to direct the light. But throughout the 18th century, as maritime traffic increased, shipwrecks multiplied. The search was under way for a stronger, more reliable light.

There is some controversy over who first placed parabolic mirrors behind flames to boost candlepower. L. Reynaud, an 18th-century chronicler and public works official, credited Swiss scientist Aimé Argand with the first installation of an apparatus using reflectors, in 1783. Argand also invented, in the 1780s, a long-burning smokeless oil lamp that removed the necessity of stoking the flames all night. But the greatest innovation was to come from Augustin Fresnel.

As a child, he was a slow learner who showed little interest in language studies or in tests of memory. By the age of 8, he could barely read. Yet his boyhood friends, for whom he studiously determined how to increase the power of popguns and bows, called him "the genius." When applied to optics, his genius proved to be real and considerable. Where others had improved existing lighthouse technology, Fresnel leapt forward by studying the behavior of light itself. His studies both advanced the understanding of the nature of light and produced the most important breakthrough in lighthouse lights in 2,000 years.

Fresnel worked out a number of formulas to calculate the way light changes direction, or refracts, while passing through glass prisms. Working with some of the most advanced glassmakers of the day, he produced a combination of prism shapes that together made up a lens. The Fresnel lighthouse lens used a large lamp at the focal plane as its light source. It also contained a central panel of magnifying glasses surrounded above and below by concentric rings of prisms and mirrors, all angled to gather light, intensify it and project it outward.

The various reflector systems installed in lighthouses during the 40 years preceding the introduction of the Fresnel lens certainly had been improvements over the open fires or candles in lantern rooms. Still, they could trap only a small percentage of the light. All prior systems paled by comparison with the Fresnel lens.


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