For thousands of years, sailors have faced a serious problem: how can you find your way along a dark coast at night? Rocks and shallow water near the shore can easily destroy a ship. The answer was the lighthouse, a tall tower with a bright light at the top. One of the most famous early lighthouses stood in Alexandria, Egypt, and it was one of the Seven Wonders of the Ancient World.
Early lighthouses used fires of wood or coal, and later oil lamps. These lights were not very strong, so ships could only see them from a short distance. In the nineteenth century, a French scientist named Augustin Fresnel designed a special lens. It is made of many rings of glass, and it collects the light and sends it out in a powerful beam.
But how do sailors know which lighthouse they are looking at? Each lighthouse has its own pattern. Some flash once every few seconds, others flash twice or show a steady light. During the day, lighthouses are recognized by their colors and stripes. Sailors check these patterns on their charts, so they know exactly where they are.
In the past, lighthouse keepers lived in or next to the towers. They climbed the stairs every evening to light the lamp and cleaned the glass. Today, most lighthouses are automatic and do not need keepers. Still, lighthouses remain important as a backup, and many of them have become museums.