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प्रश्न
The power of a lens is, −2 D. What is its focal length?
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उत्तर
Power of a lens is given by:
power p=1focal length">power p=`1/focal length`
`p=1/f`
`⇒f=1/p⇒f=1/-2f=-0.5 m`
Negative sign indicates that it's a concave lens.
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