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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

1. only d/4
2. only d/2
3. more than d/4 but less than d/2
4. less than d/4

Correct answer: only d/4

Solution

For the image to be of equal size, the object and image distances from the lens must be equal. Using the lens formula (1/f = 1/v - 1/u) and the condition that u = v = d/2, the focal length f is calculated as d/4.

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