Periscope << PEHR uh skohp >> is an optical instrument with which a person can observe objects from a distance or when vision would normally be blocked. In its simplest form, the periscope consists of a long tube with a reflecting mirror or prism at each end. These reflecting surfaces are exactly parallel to one another, and are arranged at an angle of 45 degrees with the axis of the tube. They normally enlarge the image.
Periscopes are important in submarines and tanks, and other weapons. Submarines can remain submerged at periscope depth–about 30 feet (9 meters) below the surface–while officers observe objects above the surface with the periscope. Tanks can remain closed and protected against gunfire and chemical weapons while their crews use periscopes. Periscopes enable helicopter gunships to hover behind trees or hills while the crew makes observations and shoots missiles.
Periscopes are also used in laboratories and nuclear reactors. They permit scientists and engineers to make observations without endangering themselves.
Electronic periscopes that use optical fibers are being developed. Such fibers can carry images long distances. See Fiber optics .
