It is the curvature in your eyes that makes them focus and produce a clear picture. It is the curvature in the lenses thot gives them the optical power to focus or to change the focus in your eyes -- it is this curvature and nothing else. There are no molecular changes in the glasses, there are no other physical or chemical changes. The differences in lenses are differences in curvature. A higher curvature is a higher power lens. A lower curvature is a weaker power lens.
When you go to an oculist or optometrist and he prescribes a pair of glasses, it is curvature that he prescribes, one diopter or a half a diopter of curvature in the lenses. He may prescribe spherical or cylindrical curvature. He may combine them both. He may prescribe a little spherical and a little curvature in each lens.
Lenses, to have optical power, must have either an elevated surface which is a convex curvature, or a depressed surface which is a concave curvature. If a lens has an elevation on one side and a depression on the other side it loses its optical power. If the depression is as much as the elevation it loses all optical power. It becomes a neutral lens. It is the remainder of curvature that gives the optical power. The curvature on lenses is measured by the diopter. The unit is one diopter (symbol 1.00d.).
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