A periscope is an optical device that allows an observer to see things beyond his or her line of sight. One of its classical uses is in submarines: instead of needing to surface to see what is going on at sea level, the submarine can instead stay safely submerged under the water and extend a periscope to look around. These devices are widely used in the military, but also in other fields, such as medicine and hunting.
Several crude versions of the periscope were used as far back as the 1400s, but the modern design was developed in the late 1800s, when the obvious military applications began to be realized. By World War I, they were standard issue to many troops in the trenches, and they were also used in submarine warfare.
Periscopes are made up of a long tube that contains either mirrors or prisms. The mirror or prism at one end captures the image and bends or reflects it so that it can travel down the tube to hit the mirror or prism at the other side, bending the image again so that it can meet the eye of the viewer. If the tube is extremely long, magnifying glasses may be used to enlarge it so that the viewer can see clearly. These devices can be portable, as in the case of ones used in the trenches during the First World War, or they can be fixed into a housing such as a tank or submarine.
People can use periscopes to see above their current location, allowing them to stay concealed and protected behind armor or a wall, and they can also be used to look underground and around corners. The device may be camouflaged so that it will not be readily visible to observers in the distance, ensuring that the position of the viewer will not be given away.
The science behind this tool is fairly simple, and children being introduced to basic optics are sometimes encouraged to construct one to experiment with the principles. They can also be immensely useful aids to some children's games, such as play in which children want to be able to observe a situation while remaining hidden.
Optics stores sometimes carry periscopes or kits that can be used to make them, and specialized versions are also available from medical suppliers and stores which cater to hunters. Although the device is famous for its use on submarines, most navies today actually prefer to use other imaging devices.
