Question

The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall $3m$ away by means of a large convex lens. What is the maximum possible focal length of the lens required for the purpose?

Answer 1

Given: The distance between the object and the image is $d=3m$.

The maximum focal length of the lens is given by:

$f_{max}=\frac{d}{4}$

For real image $f_{max}=\frac{3}{4}=0.75m$