Question

A bulb is located on a wall. Its image is to be obtained on a parallel wall with the help of convex lens. If the distance between parallel walls is 'd' then required focal length of lens placed in between the walls is : -

• A. only $\frac{d}{4}$
• B. only $\frac{d}{2}$
• C. more than $\frac{d}{4}$ but less than $\frac{d}{2}$
• D. less than $\frac{d}{4}$

Answer 1

Answer: (B)

only $\frac{d}{2}$

Answer 2

The convex lens is supposed to generate an actual image on the second wall from all of the bulbs that are positioned there. The lens is positioned d away from the second wall.
We now know that in order for a convex lens to properly generate an image of an item on a screen, the distance (D) between the source and the screen must equal 4f, where f is the lens's focal length.
$u = v = \frac{D}{2} = d$
$f = \frac{D}{4} = \frac{d}{2}$