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What is meant by power of a lens? What d...

What is meant by power of a lens? What does its sign indicate? State it SI unit, how is it related to the focal length of a lens?

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Power of a lens: It is defined as the reciprocal of focal length (in metres) of a lens.
`P=1/(f("in m"))`
S.I. unitof power of a lens is dioptre (D).
One dioptre: If a lens has a focal length one metre then its power is said to be one dioptre.
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