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Combinations of Lenses & Mirrors

Two lenses of...

Question

Two lenses of power -15 D and + 5D are in contact with each other. The focal length combination is:

A

-20 cm

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B

-10 cm

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C

+20 cm

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D

+10 cm

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Solution

The correct option is B -10 cm
Power of combination of two lenses,

$P = P_{1} + P_{2}$

$= - 15 + 5$

$P = - 10 D$

$\frac{1}{F} = - 10$

So focal length, $F = - \frac{1}{10} m$

$F = - 10 c m$

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