Find the power (with sign) of a concave lens of focal length $20 c m$ .

- 4 D

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- 5 D

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4 D

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5 D

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Solution

The correct option is B $- 5 D$
Power of lens is the reciprocal of the focal length
$P (d i o p t r e) = \frac{1}{f}$
Concave lens, with a virtual focus, has negative focal length and hence, negative power.
$f = - 20 c m = - 0.2 m$
Hence $P = \frac{1}{0.2} = - 5 D$

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Find the power (with sign) of a concave lens of focal length 20cm.

The correct option is B −5DPower of lens is the reciprocal of the focal lengthP(dioptre)=1fConcave lens, with a virtual focus, has negative focal length and hence, negative power.f=−20cm=−0.2mHence P=10.2=−5 D

Find the power (with sign) of a concave lens of focal length $20 c m$ .

The correct option is B $- 5 D$
Power of lens is the reciprocal of the focal length
$P (d i o p t r e) = \frac{1}{f}$
Concave lens, with a virtual focus, has negative focal length and hence, negative power.
$f = - 20 c m = - 0.2 m$
Hence $P = \frac{1}{0.2} = - 5 D$