The power P of a lens is defined to be the inverse of its focal length. In equation form, this is
P = 1f . (25.21)where f is the focal length of the lens, which must be given in meters (and not cm or mm). The power of a lens P has the unit diopters (D),
provided that the focal length is given in meters. That is, 1 D = 1 / m , or 1 m−1 . (Note that this power (optical power, actually) is not the same
as power in watts defined in Work, Energy, and Energy Resources. It is a concept related to the effect of optical devices on light.) Optometristsprescribe common spectacles and contact lenses in units of diopters.