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Question How is the light of a lighthouse magnified so that it can be seen many miles out at sea?

Answer

A lighthouse light is a concentrated beam, focused by special lenses. Because of its highly increased intensity, this beam of light can travel a very long distance.

Interior view of Fresnel lens in lantern of Split Rock Lighthouse, Lake County, MN. Jet Lowe, 1990. Prints & Photographs Division, Library of Congress.

The design of the lighthouse light as we know it today, originated at the beginning of the 18th Century. The French inventor Augustin Fresnel had correctly deduced that light was pure energy that traveled in waves, and he then spent his life developing lenses and reflectors that could capture and concentrate light.

Tillamook Rock and lighthouse, Astoria, Oregon. S.B. Crow, 1891. Prints and Photographs Division, Library of Congress.

The first lighthouse optics that he designed combined highly polished prisms with an array of lenses that captured light and concentrated it back into a main beam. The design was concentric in arrangement, funneling the light into a beam that was many times brighter than its source. This light could be seen for more than 20 miles. Fresnel’s design of concentric glass rings to concentrate light is still used today in the production of automobile headlights, traffic signals and projectors. Many of today’s lighthouses have a system of rotating lenses, and the newer ones flash off and on as a way of conserving energy.

Light House for South West Ledge, Long Island Sound. 1894. Prints and Photographs Division, Library of Congress.

Published: 11/19/2019. Author: Science Reference Section, Library of Congress

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