Fresnel lens

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Lighthouse Fresnel lens, on display at the Musée national de la Marine
How a spherical Fresnel lens collimates light

A Fresnel lens is an optical lens, which was originally developed for Lighthouses. It is named after its inventor, French physicist Augustin-Jean Fresnel. The name is pronounced /freɪˈnɛl/ (fray-NELL), the 's' is silent. Before Fresnel, Buffon and Condorcet proposed a similar design, as a way to make large burning lenses.

The design allows the construction of lenses of large aperture and short focal length without the weight and volume of material that would be required in conventional lens design. Compared to earlier lenses, the Fresnel lens is much thinner. More light can pass through. Lighthouses that use these kind of lenses can be seen from farther away.

Close-up view of a flat Fresnel lens shows concentric circles on the surface