Question

Q.38 Two lenses of power +3 and -1 diopters are placed in contact. The focal length of the combined lens is :​

Answer 1

Given:

Two lenses of power +3 D and -1 D are placed in contact.

To find:

Focal length of combined lens ?

Calculation:

The general expression of power of combination of lens is :

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

$\implies P_{net} = + 3 - 1$

$\implies P_{net} = + 2$

$\implies f_{net} = \dfrac{100}{P_{net}}$

$\implies f_{net} = \dfrac{100}{2}$

$\implies f_{net} = 50 \: cm$

So, focal length of combination of lens is 50 cm.

Answer 2

Answer:

To find:

Focal length of combined lens ?

Calculation:

The general expression of power of combination of lens is :

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

net

=P

1

+P

2

\implies P_{net} = + 3 - 1⟹P

net

=+3−1

\implies P_{net} = + 2⟹P

net

=+2

\implies f_{net} = \dfrac{100}{P_{net}}⟹f

net

=

P

net

100

\implies f_{net} = \dfrac{100}{2}⟹f

net

=

2

100

\implies f_{net} = 50 \: cm⟹f

net

=50cm

*So, focal length of combination of lens is 50 cm*