Question

C.Two thin convex lenses of focal length 25cm and 40cm are placed in contact.Find the effective power of combination.​

Answer 1

Given:

Two thin convex lenses of focal length 25cm and 40cm are placed in contact.

To find:

Effective Focal length of combination?

Calculation:

Effective Power in lens is given as:

$P_{net} = P_{1} + P_{2}$

$\implies \dfrac{1}{f_{net}} = \dfrac{1}{f_{1}} + \dfrac{1}{f_{2}}$

$\implies \dfrac{1}{f_{net}} = \dfrac{1}{25} + \dfrac{1}{40}$

$\implies \dfrac{1}{f_{net}} = \dfrac{8 + 5}{200}$

$\implies \dfrac{1}{f_{net}} = \dfrac{13}{200}$

$\implies f_{net}= 15.38 \: cm$

So, net focal length is 15.38 cm.

Answer 2

Explanation:

Given:

Two thin convex lenses of focal length 25cm and 40cm are placed in contact.

To find:

Effective Focal length of combination?

Calculation:

Effective Power in lens is given as:

P_{net} = P_{1} + P_{2}P

net

=P

1

+P

2

\implies \dfrac{1}{f_{net}} = \dfrac{1}{f_{1}} + \dfrac{1}{f_{2}} ⟹

f

net

1

=

f

1

1

+

f

2

1

\implies \dfrac{1}{f_{net}} = \dfrac{1}{25} + \dfrac{1}{40} ⟹

f

net

1

=

25

1

+

40

1

\implies \dfrac{1}{f_{net}} = \dfrac{8 + 5}{200} ⟹

f

net

1

=

200

8+5

\implies \dfrac{1}{f_{net}} = \dfrac{13}{200} ⟹

f

net

1

=

200

13

\implies f_{net}= 15.38 \: cm⟹f

net

=15.38cm

So, net focal length is 15.38 cm.