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Two thin lenses of power +5 D and -2.5 D...

Two thin lenses of power +5 D and -2.5 D are in contact. What is the focal length of the combination ?

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Combined power `P=P_(1)+P_(2)=5+(-2.5)=2.5 D`
`:.` Focal length, `f=1/P=1/2.5=10/2.5 m=(10xx100)/25 cm=40 cm`.
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