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Two lenses having focal lengths +0.20 m ...

Two lenses having focal lengths +0.20 m and -0.30 m are separated by a distance of 0.15 m. Find the resultant power of the combination.

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We know that `(1)/(f) = (1)/(f_(1))+(1)/(f_(2))- (d)/(f_(1)f_(2))`
`P= (1)/(f)= (1)/(0.20)+(1)/(-0.30)- (0.15)/((0.20)-(0.30))`
i.e., `P=(100)/(20)- (100)/(30)+(15)/(20xx0.30)`
`P=5-3.33+2.5 `
`P=4.17D`
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