A surplus Fresnel lens measuring 11" square. Fresnel may have small bends or some yellowing, but will otherwise be a functional fresnel lens.
A typical Fresnel Lens is made up of many small narrow concentric rings. Theoretically speaking, each ring can be considered as an individual small lens that bends the light path.
In fact, the curvature in each ring is approximated by a flat surface so that each ring behaves like an individual wedge prism:
The advantage of using a Fresnel lens is its compact size compared with a conventional lens. Common usage includes overhead projectors, pocket-size magnifiers and motion detectors. The word "Fresnel" comes from the person who first used this design to construct lighthouse lenses, Augustin-Jean Fresnel, in 1820. The idea of dividing a lens surface into concentric rings in order to reduce weight and size, however, can be dated back to 1748 by Georges-Louis Leclerc.