Two convex lenses of focal length 20 cm each are placed coaxially with a separation of 60 cm between them. The image of the distant object formed by the combination is at _____ cm from the first lens.

Step-by-step solution

Let L 1 be the first lens and L 2 be the second lens. The first refraction is in L 1 . Using lens formula, u 1 = ∞ ,   f 1 = 20   cm , 1 v 1 - 0 = 1 f 1   ⇒ v 1 = 20   cm Hence, the distance between the second image and the L 2 would be 60 - 20 = 40   cm . For the second lens L 2 , u 2 = - 40   cm ,   f 2 = 20   cm , 1 v 2 - 1 u 2 = 1 f 2 ⇒ 1 v 2 + 1 40 = 1 20 ⇒ 1 v 2 = 1 40 ⇒ v 2 = 40   cm So, the distance of image from first lens is