Measurement of the Refractive Power of a Lens —―Diopter‖ The more a lens bends light
rays, the greater is its ―refractive power.‖ This refractive power is measured in terms of diopters. The
refractive power in diopters of a convex lens is equal to 1 meter divided by its focal length. Thus, a
spherical lens that converges parallel light rays to a focal point 1 meter beyond thelens has a refractive
power of +1 diopter, as shown in Figure 49-8. If the lens is capable of bending parallel light rays twice
as much as a lens with a power of +1 diopter, it is said to have a strength of +2 diopters, and the light
rays come to a focal point 0.5 meter beyond the lens.
