question_answer

A convex lens of focal length 20 cm and another plano-convex lens of focal length 40 cm are placed co-axially. The plane surface of the plano-convex lens is silvered. An object O is kept on the principal axis at a distance of 10 cm from the convex lens. Find the distance d (in cm) between the two lenses so that final image is formed on the object itself.                   

Answer:

20