How does a periscope work?

Light from a scene enters the top opening and strikes a mirror or prism angled at 45 degrees, which reflects it straight down the tube. At the bottom, a second mirror or prism, parallel to the first, reflects the light out towards the viewer's eye. The result is a view from a higher position without the viewer needing to be exposed. Periscopes were used widely in the trenches of the First World War and became a defining feature of submarines, whose more advanced periscopes include lenses for magnification. Many modern submarines now use electronic masts with cameras instead of traditional optical periscopes.