Rahul find the rough focal length of a convex lens. He is trying same method to find the focal length of concave lens, will he be successful. Justify your answer with help of ray diagram.

Explanation

The rough focal length of a convex lens is obtained by forming sharp image of a very distant object on a screen. The distance of the screen from the lens gives us the rough focal length of the lens.
This method is not applicable to a concave lens, as image formed by a concave lens is virtual and it can be taken on a screen.