Question

power of combination of two lenses is a 8D if one lens has a focal length of 20cm find the focal length of the other lens

Answer 1

apply the formula p=1/f

Answer 2

Let the powers be P1, P2.

Power of their combination = P1 + P2

As given,

Focal length of P1 = 20 cm

W. K. T power of a lens = 1 / f,

Where f = focal length in metres

Thus, P1 = 1 / ( 20 / 100) ,

as 1 cm = 1 / 100 m

Therefore, P1 = 1 / 0.2 = 5 D

P1 + P2 = 8 D

5 D + P2 = 8 D

P2 = 3 D

Let the focal length of the second lens be f'

Thus, P2 = 1 / f'

3 D = 1 / f'

f' = ( 1 / 3 ) m = 100 / 3 cm ≈ 33. 33 cm