In order to make this process happen, the eye contains a lens between the retina and the pupil. The lens and the cornea, a transparent covering, then work together, which focuses the image on the retina. Vision problems occur when the image focused on the retina is focused either too far back, leading to farsightedness or too far in, leading to nearsightedness. In addition to these conditions, glasses can also work to assist with those suffering from crossed eyes, where the eyes refuse to focus together, which produces two of the same object in the field of vision, which leads to blurriness of both objects.

How glasses correct these problems is determined by what type of lens the glasses are made of. The two types of lenses are the plus and minus lens. The plus lens has a convex shape which focuses images on the retina when the eye’s lens originally focuses the image too far back. The minus lens has a concave shape, which focuses images on the retina when the eye’s lens originally focuses the image too close. Different strengths of these lenses determine how far back or how far forward the focus should be brought so that the image is displayed correctly on the retina.