pronounced "frennel" or "fraynel"
A Fresnel lens is a type of compact, multi-faceted lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses. 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 capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances.
According to Smithsonian magazine, the first Fresnel lens was used in 1823 in southwest France near the mouth of the Dordogne River; its light could be seen from more than 20 miles. Nobska’s Fresnel lens was installed 18 years later in 1841.
Fresnel produced six sizes of lighthouse lenses, divided into four orders based on their size and focal length. In modern use, these are classified as first through sixth order with the sixth order the smallest. Nobska’s Fresnel lens, pictured to the right, is a fourth order lens, a substantial size.
High-quality glass Fresnel lenses in lighthouses were considered state of the art in the late 19th and through the middle of the 20th centuries; most are now retired from service. However, the technology, in updated form, has many and varied uses including in car head lights, optical landing systems, and for production of solar energy.