Any of various tubular optical instruments that conn reflecting elements, such as mirrors and prisms, to permit observation from a position displaced from a direct line of sight.
(click for a larger image)periscope
Light enters the top lens of a periscope, reflects off a mirror set at a 45 degree angle, travels down the periscope tube, and reflects off a second mirror, also set at a 45 degree angle. Light that reflects off the second mirror exits through the bottom lens and into the eye of the user.
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