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Optical instrument (seeoptics) 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 prisms 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.
This entry comes from Encyclopædia Britannica Concise. For the full entry on periscope, visit Britannica.com.