A Brief History of Lighthouses

Lighthouses have always had two principal functions: to warn of danger from a spot that sailors could see from a safe distance both night and day, and to be guides into harbors or anchorages… These structures were often constructed under precarious circumstances by skilled builders and were maintained, often at great personal risk, by dedicated keepers.

Lighthouses in the Ancient World

The first lighthouse on record was built on the island of Pharos. Later designated one of the Seven Wonders of the ancient world…The Pharos lighthouse was the last of the six vanished Wonders to disappear (the Great Pyramid in Egypt still exists). It stood for about 1,500 years, finally falling victim to earthquakes in A.D. 1326. An Arab traveling in 1166 described the lighthouse as follows: the lowest of three stages was a square about 183 feet high with a cylindrical core; the middle stage was octagonal with 60-foot sides and a height of about 90 feet; and the third stage was circular with a height of 24 feet.1 The total height, including the foundation, was about 384 feet. It was reported to have used fire at night and a sun-reflecting mirror during the day.

The Pharos lighthouse was memorialized on Roman coins, and its name is the base for the word “lighthouse” in Spanish and Italian (faro), Portuguese (farol), and French (phare). Even in Britain before 1600, a lighthouse was called a pharos…

Technology and Lighthouses - The Light

The purpose of a lighthouse’s light is to provide a mariner at sea with a fixed point of reference to aid his ability to navigate in the dark when the shore or an offshore hazard cannot be seen directly. The distance at which such a light can be seen depends on the height and intensity of the light. The brighter the light and the greater its height above the sea, the farther it can be seen. Of course, when the weather is bad—with rain, snow, or fog—visibility can be greatly reduced…

Fresnel Lenses

Working in France, Augustin Fresnel developed lenses that enveloped a light source in all directions in what has been described variously as a “barrel,” “glass keg,” or “gigantic beehive of prisms.” By combining the reflecting (light-bouncing) and refracting (light-bending) characteristics of prisms above and below the light source, with a strong magnifying lens at the level of the light source, the light was concentrated in a narrow horizontal sheet of light…

http://americanhistory.si.edu/collections/lighthouses/history.htm