They were invented by French physicist Augustin-Jean Fresnel in 1821. Fresnel lenses focus nearly 98% of the rays produced by a central light source into a bright beam of concentrated light that can be seen from more than twenty miles out to sea. A Fresnel lens creates a bright beam of light using glass prisms set in a metal frame. These prisms change the direction that light is traveling in so all the light exits the lens in the same direction. The prisms do this by refracting (or bending) light and reflecting it as well. Refraction is the scientific word that is used to describe how light is bent as it enters or exits a dense transparent material (or medium) like glass at angle other than 90 degrees. Reflection occurs when light bounces off the surface of smooth, shiny material like metal, glass, or water. Reflection can occur on both the interior and exterior surface of transparent object. The reflection of a mountain range on the surface of a crystal clear alpine lake and the reflection of fish on the underside of the lakes surface are both great examples of external and internal reflection in action.
Fresnel lenses often use three types of prisms to focus light; dioptric prisms, catadioptric prisms, and convex prisms. Take a look at the illustration to see how each work independently and together in Fresnel lens.
Many lighthouses are still equipped with their historic Fresnel lenses while others have had their original optic removed and new modern-day electric beacons installed in their stead. Although introduced more than 200 years ago during the Industrial Revolution, Fresnel lens technology is still in use today. It can be found in the headlights of your car, in surgical lasers, and even in spacecraft.
Special thanks to:
The Ponce Inlet Lighthouse and Museum, FL