A Fresnel lens divides the optical action of a thick lens into stepped sections, often concentric annular elements. Each section redirects light while omitting much of the material between the working surface and the corresponding continuous thick-lens shape. This permits a large collecting aperture with less bulk.
In a lighthouse, sections redirect rays from a small source into a concentrated beam. The arrangement can include refraction and reflection, and rotating the optical assembly produces characteristic flashes. The lens redistributes available light rather than creating optical power; source extent, losses and aberrations limit the beam. The preserved Anacapa lighthouse lens illustrates the large practical optical assemblies built on this principle.[ref]
The Fresnel Lens by the National Park Service. National Park Service, Anacapa lighthouse Fresnel lens
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