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?

Given:

Distance between the object and the image, d = 3 m

Maximum focal length of the convex lens = f_max
For real images, the maximum focal length is given by,

f_max = d/4 …(1)

Where,

f_max = maximum focal length

d = distance between object and image

By putting the values in equation (1) , we get,

f_max = 0.75 m

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