A convergent lens of focal length 8cm combined with a diverging lens of power -10D. Calculate the focal length of the combination.

Academic Physics NCERT Class 10

Given, focal length of the convergent lens (convex lens) = f = 8cm = 0.08m.

Therefore, its power is:

$P=\frac{1}{f(meters)}$

$P=\frac{1}{0.08}$

$P=\frac{100}{8}$

$P=12.5D$

As the power of the diverging lens (concave lens) is -10D, therefore, the power of the combination is $=12.5D+(-10D)=12.5D-10D=2.5D$

The focal length of the system of lens

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

$f=\frac{1}{2.5}$

$f=\frac{10}{25}$

$f=+0.4m$

$f=40cm$

Hence, the focal length of the combination will be $40cm$.

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Updated on: 10-Oct-2022

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