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Two thin lenses of power + 6 D and - 2 D...

Two thin lenses of power `+ 6 D and - 2 D` are in contact. What is the focal length of the combination ?

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`P = P_1 + P_2 = 6 - 2 = 4 D`
`F = (100)/(P) = (100)/(4) = 25 cm`.
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