Question

For an object placed at distance 20 cm in front of a convex lens, the image is at 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.

Hint: As the object distance = image distance, the object must be kept at 2f.

Therefore, 2f = 20 cm or f = 10 cm.