How contact lenses work to correct vision is the same way eyeglasses do: they alter the direction of light rays to focus light properly onto the retina.
If you are nearsighted, light rays focus too early within your eye — they form a focus point in front of the retina instead of directly on it. Contact lenses and eyeglasses correct nearsightedness by diverging light rays, which reduces the eye's focusing power. This moves the eye's focus point backward, onto the retina where it belongs.
If you are farsighted, your eye does not have adequate focusing power — light rays fail to form a focus point by the time they reach the retina. Contact lenses and glasses correct farsightedness by converging light rays, which increases the eye's focusing power. This moves the eye's focus point forward, onto the retina.
Contact lens and eyeglass lens powers are expressed in diopters (D). Lens powers that correct nearsightedness start with a minus sign (–), and lens powers that correct farsightedness start with a plus sign (+).
So why are contact lens so much thinner than eyeglass lenses?
In large part, it's because contact lenses rest directly on the eye, instead of roughly a half-inch (12 millimeters) away from the eye's surface like eyeglass lenses.
Because of their proximity to the eye, the optic zone of contact lenses (the central part of the lenses that contains the corrective power) can be made much smaller than the optic zone of eyeglass lenses.
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