periscopen

A periscope is an optical instrument that uses a system of mirrors, prisms, or lenses to observe an object that may not be visible directly. Typically, it allows for observation over, around, or through an obstacle. The most common application is in submarines, where it enables the crew to see the surface of the water while remaining submerged. Naval warships also use periscopes for situational awareness during combat.

The basic principle of a periscope involves two parallel mirrors or prisms set at a 45-degree angle to the line of sight. Light from the object enters the upper mirror, reflects down to the lower mirror, and then reflects to the observer's eye. This effectively creates a bent line of sight, allowing the observer to look "around a corner." Modern periscopes may incorporate telescopic lenses to magnify the distant object and can also include electronic sensors for infrared or low-light vision. The development of the periscope dates back to the 17th century, with significant advancements made during wartime to improve their functionality and range.