The Fresnel Lens

http://lighthousegetaway.com/lights/fresnel.html

Stolen from Culverin2, a look at the invention of lighthouse lenses.

In 1822 a French Physicist named Augustin Fresnel invented a lens that would make his name commonplace along the seacoasts of Europe and North America. It looked like a giant glass beehive, with a light at the center. The lens could be as tall as twelve feet, with concentric rings of glass prisms above and below to bend the light into a narrow beam. At the center the lens was shaped like a magnifying glass, so the concentrated beam was even more powerful. Tests showed that while an open flame lost nearly 97% of its light, and a flame with reflectors behind it still lost 83% of its light, the fresnel lens was able to capture all but 17% of its light. Because of its amazing efficiency, a fresnel lens could easily throw its light 20 or more miles to the horizon.

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