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

20 c m

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10 c m

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+ 20 c m

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+ 10 c m

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Solution

The correct option is A $10 c m$
effective power is $- 15 D + 5 D = 10 D$

focal length is $\frac{1}{10} \times 100 = 10$

option $B$ is correct

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