Question

Two thin convex lenses of focal lengths 20cm and 25 cm are placed
coaxially 5 cm apart. Their equivalent focal length is?
​

Answer 1

Explanation:

Given: f

1

=20cm and f

2

=25cm

The focal length of the combination of lenses:

⇒

F

1

=

f

1

1

+

f

2

1

⇒

F

1

=

20

1

+

25

1

⇒F=

9

100

cm=

9

1

m

Effective power of the lens, P=

F

1

=

1/9

1

=9D