A Fresnel lens is a compact, lightweight redesign of a conventional lens - an invention attributed to French physicist Augustin-Jean Fresnel. Where a conventional lens may be thin in places and thick in others, the Fresnel lens has rectangular chunks of "zone" - rings - of the thick parts of the lens, removed to give a flatter (and so lighter) lens performing the same function. A small redesign of the curvature of each chunk may be necessary to compensate for the thinner material.
A conventional lens design has a large amount of glass (coloured red) which can be removed, with little effect on the function of the lens.
Fresnel lenses are often used in lighting (where thick lenses can crack under the heat), in flash guns, projectors and in focusing screens - where flatness is an advantage but the highest optical is not required. Lighthouse lenses are usually of fresnel design, and made in many small prismatic sections - to save weight and make construction simpler.
Fresnel lenses can be recognised by their ring pattern, and often by their lack of thickness.