Question

Parallel beam of light is incident on a system of two convex lenses of focal lengths f₁ = 20 cm and f₂ = 10 cm. What should be the distance between the two lenses so that rays after refraction from both lenses get un-deviated?

(A) 60 cm 

(B) 30 cm 

(C) 90 cm 

(D) 40 cm

Answer 1

The correct option is (B) 30 cm.

Explanation:

The image formed by first lens will lie at its second lens focus. This image will act as an object for the second lens. For the rays to become parallel after passing through the second lens, the object for second lens should lie on its first focus. Thus the distance between the two lenses will be equal to sum of their focal lengths.