Ray Optics and Optical Instruments - Result Question 40

41. A body is located on a wall. Its image of equal size is to be obtained on a parallel wall with the help of a convex lens. The lens is placed at a distance ’ d ’ ahead of second wall, then the required focal length will be

[2002]

(a) only d4

(b) only d2

(c) more than d4 but less than d2

(d) less than d4

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Answer:

Correct Answer: 41. (b)

Solution:

  1. (b) Using the lens formula 1f=1v1u

Given v=d, for equal size image |v|=|u|=d

By sign convention u=d

1f=1d+1d or f=d2



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