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:
Answer & explanation
Correct answer: option 2
Here $f_{eq}=∞$
$⇒\frac{1}{f_{eq}}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1f_2}=0$
$⇒d=f_1+f_2$
$⇒d=15cm$