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A beam of light travelling parallel to t...

A beam of light travelling parallel to the principal axis of a concave lens appears to diverge from a point 20 cm behind the lens after after passing through the lens. Find the power of the lens.

Text Solution

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By definition, the point from where the bean appears to diverge is the focus of the lens. Thus, the focal length is 20 cm. As it is a concave f is negative.
`f=-20 cm = -0.2 m.`
The power is `P=1/f=1/(-0.2 m)=-5 D`.
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