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Fresnel lens

A lightweight and flat equivalent of a lens, consisting of concentric rings, each consisting of an element of a simple lens. It is often used in lighthouses because of its thinness and lightness. The principle of the Fresnel lens is used, for example, by heliostats in central tower solar power plants. Each flat mirror is oriented differently, but together they concentrate the sunlight into a focal point like a giant concave mirror.