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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.


Any of various tubular optical instruments that contain reflecting elements, such as mirrors and prisms, to permit observation from a position displaced from a direct line of sight.

per′i·scop′ic (-skŏp′ĭk), per′i·scop′i·cal (-ĭ-kəl) adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(of a lens) having a wide field of view
ˌperiˈscopically adv
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014


(ˌpɛr əˈskɒp ɪk)

1. (of certain lenses in special microscopes, cameras, etc.) giving distinct vision obliquely, or on all sides.
2. pertaining to periscopes or their use.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.