CDS (I) 2020 General Knowledge · Question 10 of 120

Correct — A, parallel to each other. A simple periscope is a tube with a plane mirror at each end, and each mirror is tilted at 45 degrees to the axis of the tube. Light from the scene strikes the top mirror, turns through 90 degrees, runs down the tube, meets the bottom mirror and turns through 90 degrees again into the observer's eye. Two turns of 90 degrees in opposite senses bring the ray back to its original direction, and that only works if the two mirror surfaces are parallel to each other. The 45 degrees in the design describes each mirror's tilt against the tube — not the angle between the two mirrors, which is zero.

• (b)perpendicular to each other — Two mirrors at right angles send a ray straight back along the direction it came from, which is how a corner reflector works. That is useful for reflecting a signal to its source, not for looking over an obstacle.
• (c)at an angle of 60° with each other — Mirrors inclined at 60 degrees produce five images of an object between them and are the arrangement behind a kaleidoscope. A periscope needs one image, correctly oriented.
• (d)at an angle of 45° with each other — The designed 45 degrees is the tilt of each mirror against the tube, not the angle between the mirrors. Reading that number off the wrong pair of surfaces is what this option is built to catch.

A periscope lets an observer see over or around an obstruction. In its simplest form it is an outer case with a plane mirror at each end, set parallel to each other and each at 45 degrees, so that the light path is displaced sideways without being turned over. Because each mirror inverts the image laterally and there are two of them, the second reflection undoes the first and the view arrives the right way round.

Every option here mentions an angle, and the question is which pair of surfaces the angle belongs to. Drawing the light path settles it in seconds: the ray comes in horizontally, leaves the top mirror going down, and must leave the bottom mirror going horizontally again. Only equal and opposite turns achieve that, which forces the mirror faces to be parallel. Periscopes earned their reputation in the trenches of the First World War and on submarines; modern submarine periscopes replace the mirrors with prisms and lenses, and many boats have moved on again to photonic masts that carry a camera and send the picture down a cable rather than an optical tube.

• A simple periscope has two plane mirrors, parallel to each other, each inclined at 45 degrees to the tube.
• Reflection at a plane mirror is the only optical principle a simple periscope needs.
• Two reflections cancel the lateral inversion, so the image seen is erect and the right way round.
• Hippolyte Marie-Davy built the first naval periscope in 1854, a vertical tube with a small mirror at each end at 45 degrees.
• Periscopes were fitted to rifles and trench posts in the First World War so soldiers could observe without exposing themselves.

• Attaching the 45 degrees to the wrong pair of surfaces; it is mirror against tube, not mirror against mirror.
• Confusing the periscope with the kaleidoscope, whose mirrors are genuinely inclined to each other.
• Assuming refraction is involved; the simple periscope works on reflection alone.

As the mirror arrangement in a periscope, or as the optical principle on which a periscope works.

• practice — not a real PYQ

  A simple periscope works on which one of the following principles?

  • (a)Refraction of light
  • (b)Reflection of light
  • (c)Dispersion of light
  • (d)Diffraction of light

  Answer(b) Reflection of light — two plane mirrors, each at 45 degrees to the tube, turn the light path twice.

• practice — not a real PYQ

  Two plane mirrors are inclined to each other at 60°. The number of images of a point object placed between them is

  • (a)3
  • (b)4
  • (c)5
  • (d)6

  Answer(c) 5 — the count is 360 divided by the angle, minus one, so 360/60 − 1.