A periscope is an instrument for observation over, around or through an object or obstacle that prevents direct line-of-sight. In its simplest form, it consists of an outer case with mirrors at each end set parallel to each other at a 45° angle.
Periscopes are most commonly remembered for their use in Submarines, but have been used by the military in other areas such as trench warfare, gun turrets, and armored vehicles. Periscopes in the use of submarines have gradually been replaced by the photonics mast, a similar device that replaces the direct mechanical line-of-sight with digital equipment and cameras.