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 parallel beam of light falling on A leaves B as a parallel beam, then the distance ‘d’ in cm is _____
a)25
b)15
c)30
d)50




class 10 physics ​

Answer 1

15

d=f1−f2

By using the formula

f11+f11−f1f2d=F1

For the emergent beam to parallel

P=0,F=α

⇒201−51+100d=0

⇒ d=15

Answer 2

Answer:

b) 15

Explanation:

d= f1 - f2

by using the formula

1/f1 + 1/f1 - d/f1 f2 = 1/F

For the emergent beam to parallel

P=0, F=a

= 1/20 - 1/5 + d/100 = 0

= d = 15

hope it's helpful