lighthouse

Elevated structure equipped with a powerful flashing light for use as a maritime navigational aid. The light signals to ships that they are approaching land, or dangerous waters. Increasingly lighthouses are automated rather than staffed; later designs also emit radio signals that enable the ship to establish its position.

Lights may be either flashing, when the dark period exceeds the light, or occulting, when the dark period is equal to or less than the light; fixed lights are liable to cause confusion. The pattern of lighting is individually varied so that ships or aircraft can identify the lighthouse. In fog, sound signals may also be used, such as horns and sirens.

The light is magnified by one of two methods: catoptric, which uses reflection; dioptric, which uses refraction. Modern lighthouses are catadioptric, a combination of the two.