<prompt> Focal length of lens is 0.75 m, what will be its refreactive power -0.75D1.5D1.25 D1.33 D  <response>: The most convenient system for ophthalmic purpose is the one which takes a lens with focal distance of 1 meter as standard. Such a lens has a power of 1 dioptre D. Thus , simple formula to calculate the refreactive power of a lens is - 1 Refractive power D --- Focal length meter 1 In this question refractive power ---1.33D 0.75 Reference : Parson;s 22nd edition page 47 