define power of alens and write its SI units. The focal length of a lens is -50 cm. Find the power and nature of the lens.

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Solution

Power of a lens is defined as the reciprocal of its focal length. i.e. P=1/f
S.I. unit - dioptre(D).
Dioptre is the unit of power of a lens whose focal length is in metres.

Now, given focal length -50 cm.
-50cm=-0.50 m

as P=1/f,

So, P=1/-0.50=-2

Power of the lens is - 2 D and nature of the lens is concave as the focal length is -ve.

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