Analyse the following observation table showing variation of image distance (v) with object distance (u) in case of a convex lens and answer the questions that follow, without doing any calculation :

What is the focal length of the convex lens ? Give reason to justify your answer.

Explanation

The focal length of the convex lens can be calculated from S.No. 3 as when an object is placed at a distance from the convex lens its image is formed on the other side of the lens at the same distance from the lens. So, the radius of curvature is 40 cm. Thus, focal length is half the radius of curvature i.e., + 20 cm.