Two lenses whose powers are $5$ D and $- 3$ D are kept in contact. Their combined focal length is?

25

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50

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75

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1

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Solution

The correct option is D $50$ cm
Given : $P_{1} = 5 D$ and $P_{2} = - 3 D$
Power of combination of lenses $P = P_{1} + P_{2}$
$⟹ P = 5 - 3 = 2 D$
Combined focal length $F = \frac{1}{P}$
$⟹ F = \frac{1}{2} m = 0.5 m = 50 c m$

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