Q 9.19 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?
As we know for real image, the maximum focal length is given by
where d is the distance between the object and the lens.
So putting values we get,
Hence maximum focal length required is 0.75.