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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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Since, `P=P_(1)+P_(2)`
`:. P=+6+(-2)=+4D`
Since, `f=1/P=1/4=0.25 m=+25 cm`.
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