Eyeglass lenses are able to correct vision because they bend light as it passes through the lens. The amount of light-bending (or refraction) that’s needed to provide good vision is determined by the eyeglass prescription provided by your eye doctor. For weaker eyes, the number in the prescription is higher, and the lenses must bend the light more to provide clear vision. Prescriptions for nearsighted people begin with a minus symbol (-). If your prescription is -5.00 diopters, for example, you are very nearsighted and need a stronger lens than someone with a -2.00 prescription.
To bend light more, stronger minus lenses require thicker edges than weaker minus lenses. It’s not unusual for a nearsighted prescription to worsen over time, which means the edges of your lenses will grow increasingly thicker with each prescription change. Fortunately, chemists have created a variety of new plastic lens materials such as 1.60-1.71 and higher these thinner lenses bend light more efficiently than the conventional plastic lenses used for eyeglasses. This means less material can be used in high-index lenses to correct the same amount of nearsightedness.