Focal length of 0.25 m power of lens is-
It is measured as reciprocal of the focal length in metres i.e. P = 1/f . 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
40 D
1/4 D
4 D
25 D
Optics and refraction