"If you cannot saw with a file or file with a saw, then you will be no good as an experimentalist."
Augustin-Jean Fresnel
French physicist Augustin-Jean Fresnel (1788-1827) was extremely interested in the properties of light. He published a number of papers on optical phenomena such as aberration and diffraction, but he is best remembered today for an invention that turned his love of physics into a practical and revolutionary device: the Fresnel lens.
Essentially he created a new type of lens that performed in the same way as a traditional one. Fresnel's design, however, was much lighter, which made it a practical option for making the large lenses needed for lighthouses. Some of the lighthouse lenses were more than 12 feet (3.6 m) tall and looked like giant beehives. The greatly improved efficiency with which these lenses could capture light meant that up to 83 percent of the light from a powerful bulb could be focused into a beam that could be seen by ships as much as 20 miles (32 km) away at sea.
Fresnel came up with his idea whilst working as a commissioner of lighthouses in Paris. The principle is simple. If you imagine a large magnifying glass, lenticular (with two convex sides) in shape, and then slice it into dozens or hundreds of concentric rings, each ring would have a different thickness, as well as a different size. Fresnel's rings were all flat on one side and had the same thickness. In order to maintain the original function of the lens— that is, its ability to focus light—he modified the individual lens rings so that the angle of the face of each ring was different. Stacking the rings back together gives you a Fresnel lens.
Today, Fresnel lenses are used in the back of some cars and vans.
