A periscope is an optical tool that uses two mirrors or prisms to redirect light through a tube. This allows the viewer to look over or around obstacles.
The simplest periscopes use a pair of mirrors set at parallel 45-degree angles. When light strikes the upper mirror, it is reflected 90 degrees downward into the tube of the periscope. When it reaches the bottom, it is reflected 90 degrees horizontally into the viewfinder. Viewers are able to observe their surroundings as if their eyes were at the level of the top of the periscope. Periscopes are common in submarines and other military vehicles to enable occupants to survey an area without exposing themselves to fire.