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?

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 as:

 f_max  =  d/4 =3/4 =0.75 m

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