structure designed to emit light to aid navigation
A lighthouse is a tall building that sends out light for use in navigation. Lighthouses are built on the coast of an ocean or lake. The lighthouse protects ships from crashing into shore, by sending the light out towards the sea. When sailors see the light, they know to avoid hitting the shore. The light usually turns in a circle so that ships see a flashing light. The light is usually covered by a Fresnel lens. This lens enables the light to travel a far distance.
One of the most famous lighthouses was the Lighthouse of Alexandria. It was on an island near the coast. That island was called Pharos. Even today, in many languages, the word for lighthouse comes from the name of the island.
Almost all lighthouses are automatic now.