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 : (1) 30 (2) 25 (3) 15 (4) 50
Option 3) d=f1−f2 By using the formula 1/f1+1/f1−d/f1f2=1/F For the emergent beam to parallel P=0,F=α ⇒1/20−1/5+d/100=0 ⇒ d=15
