Question

two thin lenses of focal lengths 20 cm and 25 cm are placed in contact. The effective power of combination is :
(A) 45D
(B) 1/9D
(C) 9D
(D) 6D

Answer 1

NET POWER IS 9D (Option C).

Given:

• The thin lens have focal length of 20 cm & 25 cm.

To find:

Power if combination of lens ?

Calculation:

First , let's calculate the individual powers:

$P1 = \dfrac{100}{f(in \: cm)} = \dfrac{100}{20} = 5D$

Similarly for the other lens:

$P2 = \dfrac{100}{f(in \: cm)} = \dfrac{100}{25} = 4D$

Now, power of combination is:

$P_{net} = P1 + P2$

$\implies P_{net} = 5 + 4$

$\implies P_{net} = 9 \: D$

So, net power is 9 D.