A prism is used in a periscope to bend and reflect light. Simple prism-based periscopes use two triangular prisms to produce an image of something that the viewer would not otherwise be able to see. Prism-based periscopes provide better images than mirror-based periscopes.
A simple periscope is constructed from a container and two right angle prisms. The prisms are inserted into either end of the periscope with a right-angle side of one prism facing a right-angle side on the other prism in a parallel manner. The other side of each right angle needs to face out of an opening in the container. The non-right angle side of each prism faces away from an opening.
Light, transmitting an image, flows through one opening and bounces off the slanted side of the associated prism at a right angle down to the other prism. There it is again reflected off the slanted side of the prism at a right angle and out through the second opening. When the periscope user points the first opening toward the place he wants to view and positions his eye at the other opening, he is able to see the transmitted image.
Periscopes have many applications, such as peering inside nuclear reactors and seeing out of armored vehicles during military maneuvers. Modern periscopes often feature lenses and more complicated set-ups to produce better images, but they all work on the basic premise of reflection.