n.
Optical instrument (see optics ) used in land and sea warfare, submarine navigation , and elsewhere to enable an observer to see the surroundings while remaining under cover, behind armour, or submerged.
A periscope includes two mirrors or reflecting prism s to change the direction of the light coming from the scene observed: the first deflects it down through a vertical tube, the second diverts it horizontally so that the scene can be viewed conveniently.