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Question

Two plane mirrors are initially inclined at 30° and they are moved apart by 30° each time, till the angle between them becomes 120°. If an object is placed symmetrically between them then find the sum of all the images formed.


  1. 20

  2. 22

  3. 21

  4. 23


Solution

The correct option is C

21


Here number of images formed during each occassion is noted and added
Number of images formed = n = (360θ  1)

Total number of images = Number of images when angle between mirrors is at 30°, 60°, 90° and 120°

n = (36030  1) + (36060  1) + (36090  1) + (360120  1)

n = 11 + 5 + 3 + 2 = 21

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