Question

A bulb is located on a wall. Its image of same size is to be obtained on a parallel wall with the help of convex lens. The lens is placed at a distance d ahead of second wall, then 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

A real image is to be formed on the 2nd wall of the bulb placed on the first wall by the convex lens. The lens is placed at a distance of d from the 2nd wall.
Now, we know that to form a real image of an object on a screen by a convex les, the distance between the source and the screen (D) should be equal to 4f, where f is the focal length of the lens.
In that case, $u=v=D/2=d$
$\therefore$ $f=D/4=d/2$
Hence, the correct answer is option (b).