Question

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

Answer 1

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.