question_answer

A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm  are kept along the same axis with the distance d between them. If a parallel beam of light falling on  A leaves B as a parallel beam, then distance d in cm will be                                                                          [MNR 1990; IIT-JEE 1985]

A)            25  

B)            15

C)            30  

D)            50

Correct Answer: B

Solution :

                   From figure it is clear that separation between lenses                    \[d=20-5=15\,cm\]