HSPT Practice Test 2

¹For centuries, a lighthouse was only as useful as the light it could throw across the water. Early towers burned wood or coal in open fires, and later keepers used oil lamps placed in front of curved metal reflectors. Even the best reflectors wasted much of the light, which scattered upward into the sky or was absorbed by the metal. On a hazy night, a ship's crew might not see the warning until the rocks were already close.

²In the early 1820s, a French engineer named Augustin Fresnel proposed a different solution. Instead of reflecting light with metal, he would bend it with glass. A single solid lens large enough to gather a lighthouse lamp's light would have been enormously thick and heavy, and it would probably have cracked from the lamp's heat. Fresnel's design divided the lens into a series of concentric rings of prisms. Each ring was angled to bend the light toward the horizon, so that the rays left the lens in a strong, nearly parallel beam.

³Because the rings did the work of a thick lens with far less glass, the new lens was lighter and thinner, and much less of the light was lost. In 1823, the first Fresnel lens was installed in the Cordouan Lighthouse at the mouth of the Gironde River in France. Its beam reached far beyond the range of the old reflector lights.

⁴Over the following decades, Fresnel lenses were installed in lighthouses around the world. They were built in several sizes, called orders, with the largest reserved for towers on dangerous coasts and the smallest used in harbors and on rivers. Many of these lenses still exist, and visitors who climb a lighthouse tower today may see one, its rings of glass glittering in the sun.

⁵Fresnel died in 1827, only a few years after his invention first shone over the sea. Yet his idea outlived him. The same ringed design appears today in some vehicle lights, in traffic signals, and in the flat plastic magnifiers sold in bookstores. Of all the inventions of the nineteenth century, none was more important to the safety of sailors.