Question

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:

• A. only $d/4$
• B. only $d/2$
• C. more than $d/4$ but less than $d/2$
• D. less than $d/4$

Answer 1

The correct option is B only $d/2$


Let $f$ be the focal length of the lens.

In case of convex lens, an image of equal size will be formed at the distance of $2f$ from the centre of the lens.

So in order to obtain the equal size of image, $d$ will be equal to $2f$

$2f=d$

$\Rightarrow f=d/2$

So, option $\left(b\right)$ is correct.