a thin optical lens of many concentric rings, having the properties of a much thicker and heavier lens: used in cameras, lighthouse beacons, etc.
See Fresnel lens in American Heritage Dictionary 4
(frə-nĕlˈ)
noun
A thin optical lens consisting of concentric rings of segmental lenses and having a short focal length, used primarily in spotlights, overhead projectors, and the headlights of motor vehicles.
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Fresnel lens
Prisms that make up a Fresnel lens bend and magnify light rays, creating a single, concentrated beam of light.