A periscope is a device that lets you see over or around obstacles by using mirrors or prisms to bend light. Imagine standing behind a tall fence but still being able to see what's happening on the other side: that's what a periscope does.

The most famous use of periscopes is in submarines. When a submarine travels underwater, the crew can't see the surface, but they need to watch for other ships or dangers. The periscope extends up above the water like a metal tube with the submarine's “eye” at the top, bouncing the image down through mirrors to a viewer below. This lets submariners see what's happening above the waves while staying safely hidden beneath them.

Periscopes were crucial military tools in both World Wars, and submarines still use advanced versions today. You can also find simple periscopes as toys or science projects, usually made from a cardboard tube with two small mirrors angled at forty-five degrees. Some tanks use periscopes too, so the crew can observe their surroundings while staying protected inside armored vehicles.