A periscope is an instrument people use to look at things from a hidden position. The device has a long tube with parallel mirrors situated at both ends at a 45-degree angle. However, some periscopes opt for prisms rather than mirrors, such as those in submarines. The military typically uses periscopes in armored vehicles and gun turrets.

The word periscope derives from two Greek words. “Peri” means “around,” and “scopus” means "to look.” Hence, a periscope has the ability to turn around in a circular manner to view objects usually above ground or on the water's surface.

Besides reflecting prisms, periscopes on submarines usually have two telescopes. A submarine features heavy-duty, thick waterproof casing that makes it sturdy enough to withstand high water pressure.

Johann Gutenberg offered a periscope to allow pilgrims to see objects over the heads of others at a religious festival in the 1430s.

When you have programmed both telescopes to have two different sizes of magnification at the same time, the magnification difference between the telescopes results in either an overall enlargement or reduction of the images.

The main purpose of the development of the periscope in a submarine was to provide a way to see above the surface while still underwater. Most navies around the world use similar instruments, but employ various designs.

Marie Davey, a French inventor, created a submarine periscope that consisted of a two mirrors a both ends that were held at a 45-degree angles and faced opposing directions. However, in 1872, prisms replaced the mirrors, according to the San Francisco Maritime Association.

The periscope uses a long tube to remain inconspicuous while operating a submarine in water. People do not notice the device even though a section of the periscope sits slightly above water.

Sailors use a standard numbering system for referencing periscopes. The periscope closest to the bow, or front of the ship, is the No. 1 periscope, and so on.