# A lens has power 4 D. Find the focal length that lens.

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A lens has power 4 D. Find the focal length that lens.

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Given Focal length (f) = 4D

P (in diopter) = $\frac{1}{f(in\,meters)}$

⇒ 4 = $\frac{1}{f(in\,meters)}$

⇒ f (in meters) = $\frac14$= 0.25m

Now 1m = 100cm

⇒ 0.25m = 25cm

Hence the focal length of lens is 25cm.