periscope

A periscope is an optical instrument that allows a viewer to see objects that are not in the direct line of sight, typically by guiding light through a tube with reflective surfaces. The classic design uses two mirrors or two prisms arranged so that light from the scene above the obstruction is reflected down the tube to an eyepiece at the opposite end. Modern periscopes may incorporate lenses for magnification and additional optics or electronic sensors to improve image quality.

Naval and other specialized uses are the most familiar. In submarines and other vessels, a periscope rises above the water surface while the operator remains in a protected compartment, enabling observation, navigation, and target acquisition without surfacing. Periscopes for military ships and tanks often include features such as rangefinders, night vision or image-intensification tubes, and sometimes cameras for recording or transmission.

Historical development began in the 19th century as a means to see from protected positions. The basic concept—reflecting light from a distant view through a tube—has been adapted for various applications, from surveying and testing to viewing over obstacles in field scenarios. Beyond military uses, periscopes have found roles in photography, science, and consumer devices, where compact, unobtrusive viewing is advantageous.

In the consumer sphere, Periscope was a social app launched by Twitter in 2015 that enabled live video streaming from mobile devices. It operated as a separate service until its shutdown in 2021, after which some functionality and members migrated to Twitter’s platforms. The term continues to describe any instrument that provides a view from a concealed or protected position rather than a specific product.