Question

Two lenses of Focal lengths 20 cm and 25 Cm are contacted together find power of contact lens so formed. answer plzzz​

Answer 1

Answer:

f  

1

​  

=20cm=0.2m;f  

2

​  

=25cm=0.25m

as power of lens is given by,

P=  

f

1

​  

 

P  

1

​  

=  

f  

1

​  

 

1

​  

=  

0.2

1

​  

=5D,

P  

2

​  

=  

f  

2

​  

 

1

​  

=  

0.25

1

​  

=4D

so,The effective power of the combination is 5D+4D=9D.

Explanation:

Answer 2

Given:

Two lenses of Focal lengths 20 cm and 25 cm are contacted together.

To find:

Power of the combination of lens.

Calculation:

The general expression for power of combination of lens is :

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

$= > \rm{P_{net} = \dfrac{1}{f1} + \dfrac{1}{f2} }$

Here the focal lengths have to put in metre unit.

$= > \rm{P_{net} = \dfrac{1}{ (\frac{20}{100} )} + \dfrac{1}{( \frac{25}{100}) }}$

$= > \rm{P_{net} = \dfrac{1}{0.20} + \dfrac{1}{0.25}}$

$= > \rm{P_{net} = 5+ 4}$

$= > \rm{P_{net} = 9 \: Dioptre}$

So, final answer is:

$\boxed{\bf{P_{net} = 9 \: Dioptre}}$