Question

2 What is power of Lens of focal length of 20cm-----​

Answer 1

Explanation:

Given: Two lens of focal length, f

1

=−20cm and f

2

=+10cm are put in combination

To find the power of the combination.

Solution:

If focal length of the combined lens is F

Use the formula,

F

1

=

f

1

1

+

f

2

1

⟹

F

1

=

(−20)

1

+

10

1

⟹

F

1

=

20

−1+2

⟹F=20cm=0.2m

So the power of the combination is,

P=

0.2

1

=+5D

Answer 2

Explanation:

focal length is 20cm

power of lens is 1/f in metres

and 100/f in centimetres

power of lens=100/20

= 5 dioptre