The uses of a periscope include observation around barriers and at times when viewing a location directly would be dangerous. Periscopes are typically used in submarines to see above the water, in tanks to see above and around the vehicle and by people attending parades and other events to see over the crowd.
Periscopes also are sometimes used for seeing into nuclear reactors and were used for seeing out of trenches in World War I. Simple versions of the optical instruments were first used in the 1600s, with submarine periscopes first used in the 1800s. In his famous 1927 trans-Atlantic flight, Charles Lindbergh carried so much fuel that he needed a periscope to see around the fuel tank of his plane.
A simple periscope is constructed from a tube with two mirrors set at a 45-degree angle in relation to the tube. When mirrors are parallel to each other, the periscope can be used to view objects in front of the viewer. If the mirrors are at right angles, the periscope is used to see objects behind the viewer, and the image seen is upside down. In complex periscopes, prisms are sometimes used instead of mirrors. Submarines use lens systems in addition to mirrors for an enlarged view.