Question

Two lenses of power 20d-10d are in contact with each other.The focal lenght of the combination is :

Answer 1

P=P1+P2

P=20-10

= 10D

P=1/f

10=1/f

f=1/10=0.1m

i.e 10 cm

Answer 2

Given:

Two lenses of power 20D and -10D are in contact with each other.

To Find:

The focal length of the combination.

Solution:

We know that the power of the combination of lenses is given by

$P=P_1+P_2+...+P_n$,  

where $P_1,P_2,...,P_n$ are the powers of lenses $L_1,L_2,...,L_n$

So using the formula we can find the power of the combination of the two lenses.

$P=+20-10=10D$

The power of a lens is the reciprocal of its focal length. So,

   $P=\frac{1}{f}$

⇒ $f=\frac{1}{P}$

⇒ $f=\frac{1}{10} =0.1m$

Hence, the focal length of the combination of the lenses is equal to 10cm.