The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall 4 m away by means of a large convex lens. The maximum possible focal length of the lens required for this purpose will be

Step-by-step solution

Let a large convex lens is placed between two walls at a distance x from the wall on which an electric bulb is fixed. Using 1 f = 1 v - 1 u = 1 4 - x - 1 - x                   ∵ u = - x ,   v = 4 - x = x + 4 - x 4 - x x = 4 ( 4 - x ) ( x ) o r   f = 4 - x x 4               … … i Now magnification m = v u = 4 - x x ⟹   1 = 4 - x x ⟹ x = 4 - x ⟹ 2 x = 4 ⟹ x = 2   m From eq.