Question

 A convex lens A of focal length 20 cm & a concave lens B of focal length 5 cm are kept along the same axis with a distance d between them. What is the values of d if a parallel beam of light incident on A leaves B as a parallel beam 

(a) 15 cm 

(b) 30 cm 

(c) 10 cm 

(d) none

Answer 1

Correct Option :- (a) 15 cm

Explanation :-

As the incident beam is parallel in absence of concave lens it will form an image at a distance v from it such that 

(1/v) – (1/-∞) = (1/20) ⇒ v = 20 cm = f 

Now since d is the distance between convex & concave lens. The distance of image I from concave lens will be (20-d). Since the image I will act as an object for concave lens 

which forms its image at ∞, so 

(1/v)- (1/u) = (1/f) (1/∞) – [1/(20-d)] = 1/(-5) , 

20-d =5 ⇒ d = 15 cm