<prompt> Focal length of 0.25 m power of lens is ?40 D14 D4 D25 D  <response>: Ans. is c i.e., 4 D Power of a lens P is defined as the ability of the lens to converge a beam of light falling on the lens. o For a converging convex lens the power is taken as positive and for a diverging concave ? ens power is taken as negative. It is measured as reciprocal of the focal length in metres i.e. P 1f. The unit of power is dioptre D. One dioptre is the power of a lens of focal length one metre. Therefore,Power of lens 1f 10.25 4D Thus, A lens with a focal length of 0.25 m will be four times as strong as one with a focal length of 1 m: the refractive power of such a lens is therefore 4 dioptres 4 D. 