(via Fresnel lens | Wikipedia)
A Fresnel lens is a type of composite compact lens originally developed by French physicist Augustin-Jean Fresnel (1788–1827) for lighthouses.[1][2] It has been called "the invention that saved a million ships."[3]
The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances.
Source: Wikipedia