Question

The equivalent focal length of two thin lenses in contact is 20cm if power of one lens is -3d find the focal lengthof the lens

Answer 1

The answer is given below .....

Answer 2

The focal length of lens 2 is 12.5 cm.

Explanation:

Given that,

The equivalent focal length of two thin lenses in contact is 20 cm, f = 20 cm

The power of lens 1, $P_1=-3\ D$

Power if lens is given by :

$P_1=\dfrac{1}{f_1}$

$f_1=\dfrac{1}{P_1}$

$f_1=\dfrac{1}{-3\ D}=0.33\ m = -33.4\ cm$

The power of the combination is given by :

$P=P_1+P_2$

Here, $P_2$ is the power of lens 2.

So,

$\dfrac{1}{f}=\dfrac{1}{f_1}+\dfrac{1}{f_2}$

$\dfrac{1}{f_2}=\dfrac{1}{f}-\dfrac{1}{f_1}$

$\dfrac{1}{f_2}=\dfrac{1}{20}-\dfrac{1}{(-33.4)}$

$f_2=12.5\ cm$

So, the focal length of lens 2 is 12.5 cm. Hence, this is the required solution.

Learn more,

Power of a lens

https://brainly.in/question/8480629