Question

A luminous object is separated from a screen by distance $d$. A convex lens is placed between the object and the screen such that it forms a distinct image on the screen. The maximum possible focal length of this convex lens is:

• A. $4d$
• B. $2d$
• C. $\frac{d}{2}$
• D. $\frac{d}{4}$

Answer 1

Answer: (D)

$\frac{d}{4}$

From the displacement method experiment of lens, we know that the distance between the source and the screen should be greater than four time the focal length of the convex lens, i.e., $d>4f$
So, the maximum possible focal length of this convex lens is $\frac{d}{4}$.