A periscope is an optical instrument for viewing objects that are not directly in the observer's line of sight. Periscopes in submarines enable crew members to see above the water without bringing the submarine to the surface. The periscope on an army tank enables the men inside to view the area surrounding the tank while remaining protected. Scientists use periscopes to view dangerous experiments, such as nuclear reactions, from a safe position.

The simplest type of periscope consists of two reflecting pieces, either mirrors or prisms, set into a tubelike apparatus. The reflecting surfaces may be parallel or at right angles, depending on whether the observer is looking forward or to the rear. Light rays entering the tube are reflected through the body of the tube to the eye of the observer.

The submarine periscope is more complicated in design. The length of the tube may be as much as 50 times its diameter. The image is transmitted through the tube by a system of prisms and lenses that provide a wide field of view.

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