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 and lens is ${}^{\prime }{d}^{\prime }$ 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 or equal to $\frac{d}{4}$

Answer 1

The correct option is B Only $\frac{d}{2}$

R.E.F image

A real image is to be formed on the $2^{nd}$ all of the bulb placed on the first wall by the convex lens. The lens is placed at a distance of d from the $2^{nd}$ wall

Now, we know that to form a real image of an object on a screen by a convex lens,

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 caus,

$u=v=D/2=d$

$f=D/4=d/2$