Question

A body is located on a wall. It's image of equal size is to be ibtained 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 and less than d/2 4. Less than d/4 (Ans is 2. But please explain the reason)

Answer 1

By lens formula,
1/f=1/v - 1/u
I =0 and |v| = |u| =d
1/f = 1/d -(-1/d) = 2/d
.'. f = d/2

Answer 2

Answer:

Explanation:

Magnification has to be 1 .....

m= v / u

1 = v / u

V =U = d

1/f = 1/v - 1/u

V=d

U= -d

f= d/2