Question

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 a distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then the distance d in cm will be –

• A. 25
• B. 15
• C. 30
• D. 50

Answer 1

Answer: (B)

15

Answer 2

\ $\frac { 1 } { \infty } = \frac { 1 } { 20 } + \frac { 1 } { - 5 } - \frac { d } { 20 ( - 5 ) }$

Ž 0 =

\ $\frac { d } { 100 } = \frac { 1 } { 5 } - \frac { 1 } { 20 } = \frac { 20 - 5 } { 100 }$

\ d = 15 cm