Question

What is equivalent lens? The equivalent focal length of two thin lenses in contact is 20cm.If power of one lens is(-3D).Find the focal length of other lens

Answer 1

Answer:

What is equivalent lens ? The equivalent focal length of two thin lenses in contact is 20cm if Power of one lens is (-3D) find the focal length of other lens.

Answer 2

The focal length of equivalent lenses is 5 cm.

Explanation:

Given that,

The combination of two or more lenses is called equivalent lens.

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

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

To find,  

The focal length of other lens.

Solution,

In case of combination of lens, the equivalent power is given by :

$P=P_1+P_2$

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

$\dfrac{1}{20}=(-3)+\dfrac{1}{f_2}$

$f_2=0.325\ m$

or

$f_2=32.5\ cm$

So, the focal length of other lens is 32.5 cm.

Learn more,

Combination of lenses

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