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A flat form of lens that uses concentric rings of glass instead of one large piece to focus light.

Each ring can be considered a fragment of a more complete hypothetical lens, usually convex. When all lens fragments have the same focal length, they behave as one lens.

Fresnel lenses were first created for lighthouses. The problem was that they wanted a large lens with a relatively short focal length so that the lens could be placed close to the light source. However, such a lens would have to be very thick, and the ideal traditional lens could easily end up weighing more than a ton. Because of this, it would costing a fortune in materials and labor to transport and install in a tall, thin tower. Plus, if it got cracked, you had to replace the whole shebang.

The advantages of fresnel lenses were that they were lightweight compared to their single element cousins, and the increased cost in engineering was more than offset by the decrease in material required. The modularity was a plus too, as a cracked element in the structure would only lead to a small decrease in performance, and was (relatively) cheaply replaced.

Today, fresnel lenses can be simply stamped out of thick sheets of glass or plastic. You can see plastic fresnel lenses stuck to the back of some minivans in an attempt to remove the blind spots. Glass lenses are reserved for more high-end applications, such as stamp collecting and jewelry making.