Two lens of focal length $- 20$ cm and $+ 10$ cm are put in combination, find the power of the combination.

- 1

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- 2

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+ 5

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+ 2

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Solution

The correct option is B $+ 5$ D
Given: Two lens of focal length, $f_{1} = - 20 c m$ and $f_{2} = + 10 c m$ are put in combination
To find the power of the combination.
Solution:
If focal length of the combined lens is $F$
Use the formula,
$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} ⟹ \frac{1}{F} = \frac{1}{(- 20)} + \frac{1}{10} ⟹ \frac{1}{F} = \frac{- 1 + 2}{20} ⟹ F = 20 c m = 0.2 m$
So the power of the combination is,
$P = \frac{1}{0.2} = + 5 D$

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Two lens of focal length −20cm and +10cm are put in combination, find the power of the combination.

Two lens of focal length $- 20$ cm and $+ 10$ cm are put in combination, find the power of the combination.