Question

The equivalent focal length of two thin contacts 20cm . If power of one lensis(-3d),find the focal length of other lense

Answer 1

The focal length of the other lens is 12.59 cm.

Explanation:

Given that,

The equivalent focal length of two thin that are in contact 20 cm. The power of one lens is (-3 D).

The equivalent focal length of two thin that are in contact is given by :

$\dfrac{1}{F}=\dfrac{1}{f_1}+\dfrac{1}{f_2}$..........(1)

Power of a lens,

$P=\dfrac{1}{f}$

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

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

$f_1=\dfrac{1}{(-3)}$

$f_1=-0.34\ m$

$f_1=-34\ cm$

The equation (1) becomes :

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

$\dfrac{1}{f_2}=\dfrac{1}{(20)}-\dfrac{1}{(-34)}$

$f_2=12.59\ cm$

So, the focal length of the other lens is 12.59 cm. hence, this is the required solution.

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