A periscope is an instrument for observation from a concealed position. In its simplest form, it consists of a tube in each end of which are mirrors set parallel to each other at 45 degree angle.
This form of periscope, with the addition of two simple lenses, served for observation purposes in the trenches during World War I. Military personnel also use periscopes in some gun turrets and in armored vehicles.
More complex periscopes, using prisms instead of mirrors, and providing magnification, operate on submarines. The overall design of the classical submarine periscope is very simple: two telescopes pointed into each other. If the two telescopes have different individual magnification, the difference between them causes an overall magnification or reduction.
Johann Gutenberg, better known for his contribution to printing technology, marketed a periscope in the 1430s to enable pilgrims to see over the heads of the crowd at the vigintennial religious festival at Aachen. Simon Lake used periscopes in his submarines in 1902. Sir Howard Grubb perfected the device in World War I. Morgan Robertson (1861–1915) claimed to have tried to patent the periscope: he described a submarine using a periscope in his fictional works.
Periscopes are extensively used in tanks, enabling drivers or tank commanders to inspect their situation without leaving the safety of the tank. An important development, Gundlach's periscope, had a rotating top, allowing a tank commander to obtain 360 degree view without moving. The design was first used in the Polish 7-TP light tank (produced from 1935 to 1939). Shortly before World War II it was given to the British  and was used in most tanks of World War II, including the British Crusader, Churchill, Valentine, and Cromwell and the American Sherman. The USSR later copied the design and used it extensively in its tanks (including the T-34 and T-70); Germany also made and used copies.
Periscopes allow a submarine, when submerged at a shallow depth, to search for targets and threats in the surrounding sea and air. When not in use, a submarine's periscope retracts into the hull. A submarine commander in tactical conditions must exercise discretion when using his periscope, since it creates a visible wake and may also become detectable by radar, giving away the sub's position.
The Frenchman Marie Davey built a simple, fixed naval periscope using mirrors in 1854. Thomas H. Doughty of the US Navy later invented a prismatic version for use in the American Civil War of 1861-1865.
The invention of the collapsible periscope for use in submarine warfare is usually credited to Simon Lake in 1902. Lake called his device the omniscope or skalomniscope. There is also a report that an Italian, Triulzi, demonstrated such a device in 1901, calling it a cleptoscope.
In another early example of naval use of periscopes, Captain Arthur Krebs adapted two on the experimental French submarine Gymnote in 1888 and 1889. Perhaps the earliest example came in 1888 from the Spanish inventor Isaac Peral on his submarine Peral - developed in 1886 but launched on September 8, 1888. Peral's fixed, non-retractable periscope used a combination of prisms to rely the image to the submariner, but his submarine pioneered the ability to fire live torpedoes while submerged. Peral also developed a primitive gyroscope for his submarine navigation.
As of 2009 modern submarine periscopes incorporate lenses for magnification and function as a telescope. They typically employ prisms and total internal reflection instead of mirrors, because prisms, which do not require coatings on the reflecting surface, are much more rugged than mirrors. They may have additional optical capabilities such as range-finding and targeting. The mechanical systems of submarine periscopes typically use hydraulics and need to be quite sturdy to withstand the drag through water. The periscope chassis may also be used to support a radio or radar antenna.
Submarines traditionally had two periscopes: a navigation or observation periscope and a targeting, or commander's, periscope. Early navies originally mounted these periscopes in the conning tower, one forward of the other in the narrow hulls of diesel-electric submarines. In the much wider hulls of recent US Navy submarines, the two operate side-by-side. The observation scope, used to scan the sea surface and sky, typically had a wide field of view and no magnification or low-power magnification. The targeting or "attack" periscope, by comparison, had a narrower field of view and higher magnification. In World War II and earlier submarines it was the only means of gathering target data to accurately fire a torpedo, since sonar was not yet sufficiently advanced for this purpose (ranging with sonar required emission of an electronic "ping" that gave away the location of the submarine) and most torpedoes were unguided.
21st century submarines do not necessarily have periscopes. The United States Navy's Virginia-class submarines instead use photonics masts, which lift an electronic imaging sensor-set above the water. Signals from the sensor set travel electronically to workstations in the submarine's control center. While the cables carrying the signal must penetrate the submarine's hull, they use a much smaller and more easily sealed—and therefore less expensive and safer—hull opening than those required by periscopes. Eliminating the telescoping tube running through the conning tower also allows greater freedom in designing the pressure hull and in placing internal equipment.