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

Distance between the object and the image, d=3m
Maximum focal length of the convex lens= ƒmax
For real images, the maximum focal length is given as:
$f_{max}=\frac{d}{4}$
= $\frac{3}{4}$

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