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As light rays pass through a lens with power, the rays are bent or refracted. In a lens with a plus power, the light rays converge or are refracted toward one another. The point at which the light rays converge is called the focal point and in a plus lens, is behind the lens surface. In a lens with a minus power, the light rays diverge or are refracted away from one another. If these rays are extrapolated or traced back toward the light source, the lines will converge and form a focal point in front of the lens surface.

The lens power is relative to the focal distance or the distance between the focal point and the lens. More specifically, lens power is the reciprocal of the focal distance in meters. Lens power is expressed in diopters (D).

Power = 1 / Focal Distance

As explained above, if the light rays converge, the focal distance is expressed with a positive value resulting in a positive or plus power. If the light rays diverge, the focal distance is expressed with a negative value resulting in a negative or minus power.

Example:

Light rays pass through a lens and converge 0.50 m from the lens What is the power of the lens?

Power = 1 / Focal Distance

Power = 1 / 0.50 m

Power = 2.00 D