A periscope is an optical device that allows a person to see objects that are not in their direct line of sight. It typically consists of a long tube with mirrors or prisms at each end. When light enters the top of the tube, it reflects off the mirrors, allowing the viewer to see over obstacles, such as walls or the surface of the water. Periscopes are commonly used in submarines, where they help crew members observe the surface while remaining submerged. They can also be found in some military applications and even in children's toys, providing a fun way to explore the world from different angles.