A periscope is the retractable optical instrument used by a submarine to see above the surface.
A conventional submarine periscope uses mirrors or prisms inside a narrow vertical tube. When raised, its upper lens reaches above the water while the officer looks through an eyepiece inside the pressure hull, allowing observation without fully surfacing.
Submarines commonly use attack periscopes for targeting and observation periscopes for navigation or surveillance. Modern boats may supplement or replace traditional optical systems with photonics masts carrying cameras and sensors.
A periscope is not the same as sonar, which uses sound waves to detect objects, or a snorkel, which admits air for diesel engines. The word comes from Greek roots meaning “around” and “to look.”