What is a periscope used for?

A periscope: An optical instrument that uses a series of prisms, lenses and mirrors to reflect images through a tube. Most often associated with submarines but they can also have other practical uses.
A periscope works by using mirrors to bounce light from one mirror to the other. A normal periscope uses two mirrors at 45 degrees angles to the direction you want to see. The light bounces from one to the other and then to your eye. A periscope helps you to see around corners and over walls, they are also used on submarines so that they out and see around things.