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Power and Thin Lenses in Contact

Two lenses of...

Question

Two lenses of power 12 and -2 dioptre are placed in contact. The focal length of the combination is

A

10cm

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B

15cm

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C

12cm

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D

8.33cm

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Solution

The correct option is A 10cm
Power of the combination $P = P_{1} + P_{2} = 12 - 2 = 10 D$
$∴$ focal length of the combination
$F = \frac{100}{P} = \frac{100}{10} = 10 c m$

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