<prompt> Focal length of 0.25 m power of lens is-40 D14 D4 D25 D  <response>: 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. The focal length f of a lens is the distance between the optical centre and the principal focus. Power of a lens P is defined as the ability of the lens to converge a beam of light falling on the lens Reference : A K KHURANA OPHTALMOLOGY,Edition 4, page-23 