How does a lighthouse work?

A lamp on its own shines in every direction at once, which sounds generous and is actually the problem. Put one at the top of a tower on the coast and most of it goes straight up into empty sky, or straight down into the water. Only a thin band travels out flat toward the sea, which is the one direction that matters. A light like that does its job with a sliver of itself.

About two hundred years ago a French scientist named Augustin Fresnel worked out the fix. He surrounded the lamp with rings of glass, circles inside circles, stacked into a shape people describe as a glass beehive. The rings above catch the light heading skyward and bend it back down until it lies flat. The rings below catch the light heading into the water and bend it up. Every ring aims its share at the same place, the horizon. Before him the best lighthouses used about half their light. With the rings they used nearly all of it, and a flame you could have covered with your hands became something a ship could see twenty miles out.

There is a second trick, and it is not about brightness at all. There is never just one lighthouse on a coast, and in the dark one light looks like any other, so each is given its own pattern of flashes. That pattern is called the light's character, and charts list every one of them. Count what you can see, look it up, and you know which lighthouse it is, which tells you exactly where you are. The light at Pigeon Point in California has flashed once every ten seconds for more than a hundred and twenty-five years, and it keeps doing it on the nights when there is nobody on the water at all.