Question

For an object placed at a distance 20 cm in front of a convex lens, the image is at a distance 20 cm behind the lens. The focal length of convex lens is :

(a) 20 cm (b) 10 cm (c) 15 cm (d) 40 cm

Answer 1

The focal length of the convex lens is 10 cm.
As the object distance = image distance, the object must be kept at 2f. Therefore, 2f = 20 cm or f = 10 cm.
option b is correct