lighthouse

Introduction

lighthouse, towerlike structure erected to give guidance and warning to ships and aircraft by either visible or radioelectrical means. Lighthouses were long built to conform in structure to their geographical location. Until the beginning of the 19th cent. tallow candles, coal fires, and oil lamps were used as illuminating agents; coal gas followed, to be succeeded by acetylene. Electricity was used for the first time at South Foreland Light, England, in 1858. Other 19th-century innovations were rapidly revolving lights, the incandescent oil-vapor light, fog bells, whistles, sirens, diaphones (fog signals similar to sirens), and the Fresnel lens (used to focus the beam).

In modern lighthouses there are three kinds of lighting systems: the catoptric system, in which rays of light are reflected from silvered mirrors to form a parallel beam visible at a distance; the dioptric, or refractive, system, in which the rays pass through optical glass and are refracted as they enter and emerge from it; and the catadioptric system, in which rays are both refracted and reflected. Increased use of radio beams and radar has made the conventional lighthouse obsolete.

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