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

Answer:

We are given that –
Distance between the object and the image is d = 3 m

Maximum focal length of the convex lens is = $f_{max}$
The maximum focal length, for real images is given as:

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

= $\frac{3}{4}=0.75m$

Hence, the maximum possible focal length of the convex lens is 0.75 m for the required purpose.