Periscope
Periscope, an optical device used for viewing objects that are out of the line of sight. Periscopes are used in tanks and submarines where it is necessary to see from a protected position. They have also been used for viewing the interiors of nuclear reactors. Simple periscopes are sometimes used by onlookers at parades, for seeing over the heads of the crowd.
A simple periscope consists of a tube and two mirrors. The mirrors are set at angles of 45° to the length of the tube. If the mirrors are parallel to each other, the periscope "looks" forward and the object seen is rightside-up. If the mirrors are set at right angles to each other, the periscope "looks" backward and the object seen is upsidedown. In high-quality periscopes, reflecting prisms are often used instead of mirrors. Submarine periscopes have systems of lenses in addition to the mirrors or prisms to provide an enlarged view of the object or scene.