A periscope is an instrument for observation over, around, or through an object, obstacle, or condition that prevents direct line-of-sight observation from an observer's current position. Complex periscopes using prisms or advanced fiber optics instead of mirrors and providing magnification operate on submarines and in various fields of science. The overall design of the classical submarine periscope is very simple: two telescopes pointed at each other. If the two telescopes have different individual magnifications, the difference between them causes an overall magnification or reduction.

Source: Wikipedia