How Do Contacts Work?
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 lenses 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 (typical positioning of 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.
In fact, the optic zone of eyeglass lenses is the entire lens surface. The optic zone of contact lenses is only a portion of the lens, which is surrounded by peripheral fitting curves that do not affect vision.
It's something like looking out a small window in your house: If you are standing very close to the window, you have a large, unobstructed view of the outdoors. But if you are standing across the room from the window, your view outside is very limited — unless you have a much larger window.
Because contact lenses rest directly on the cornea, their optic zone only needs to be roughly the same diameter as the pupil of the eye in low-light conditions (about 9 millimeters). In comparison, to provide an adequate field of view, most eyeglass lenses are greater than 46 mm in diameter. This larger size makes eyeglass lenses much thicker than contact lenses.
Also, eyeglass lenses must be made much thicker than contact lenses to keep them from breaking upon impact. Lenses for nearsightedness in eyeglasses must have a minimum center thickness of 1.0 mm or greater to meet impact resistance guidelines.
Contact lenses can be made much thinner. In fact, most soft contact lenses for nearsightedness have a center thickness that is less than 0.1 mm.
So, it's the combination of significant differences in wearing position, optic zone diameter and minimal thickness to ensure structural integrity that makes contact lenses much, much thinner than eyeglass lenses of the same power.