Lesson 3
The Curvature in Glasses

A curve means a bend -- a bend means a curve. Most of us are familiar with curves or bends on the road. There are straight roads and curved roads. There are short curves, and long curves that are hardly noticeable. A curve is part of a circle, it is also part of a sphere, and it is also part of a cylinder. In optics, a curve is called a curvature.

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.).


The optical curvatures of lenses and of our eyes are relative curvatures. It is similar to relative humidity.

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