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Some lighthouses can be seen from over 30 miles away at sea.

A spinning lens focuses a single bulb into a beam powerful enough to warn ships miles from a dangerous coast.

2 min read
Some lighthouses can be seen from over 30 miles away at sea.
THE FULL STORY

Lighthouses are some of the simplest yet most important navigation tools humans ever invented. A bright light at the top of a tall tower warns ships of rocks, reefs, harbors, and headlands. The earliest ones used wood fires. The Tower of Hercules in Spain has been guiding ships since Roman times - about 1,900 years - and is still working today.

The big leap came in 1822, when French physicist Augustin-Jean Fresnel invented a special lens made of concentric glass rings. The Fresnel lens grabbed light that would normally scatter and bundled it into a tight beam. A single bulb could suddenly throw a beam visible from over 30 miles at sea, replacing the bonfires keepers used to feed every night.

Most lighthouses also flash in a unique pattern so sailors can tell them apart. One might flash every 4 seconds, another every 12, with longer or shorter pauses. By matching a flash pattern against a chart, a captain instantly knows which point of land they’re seeing. Today most lighthouses run automatically - the famous lonely keepers are mostly retired.

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