The Deal

• $50 for $200 value towards prescription glasses and frames

Frames start at $249, lenses start at $225 for single vision.

Corrective Lenses: Shedding Light Where It’s Needed

There’s a lot that can go wrong in the human eye, but fortunately opticians can do a lot to make up for that. Learn how with Groupon’s introduction to corrective lenses.

Light begins its transformation from electromagnetic radiation into visual information by traversing three parts of the eye. It first hits the outermost layer—the cornea—and then enters through the pupil to pass through the lens. The shape and orientation of all three elements are crucial, as they all affect how light is refracted onto the optic-nerve-connected retina at the back of the eye. If these mechanisms don’t get it right, corrective lenses can add a fourth layer to compensate.

For instance, nearsightedness is typically caused by a cornea that is overly rounded from staring at too many billiard balls, or by an eyeball that is too long. As light passes through the pupil, it focuses the light at a point before it hits the retina, creating a blurrier signal. So in this case, concave lenses work to spread the light they intercept before it hits the natural lens.

Today, sophisticated computer programs determine the exact degree of curvature necessary on each surface of the lens to approximate normal vision for eyes with various irregularities. The lenses themselves may be made of several different materials; glass, naturally, is the oldest, although its weight and fragility makes it less popular today. Plastic resin is most widely used, but injection-molded polycarbonate is a favorite of many opticians: along with being durable and light, it also filters out UV radiation and several of the dirtier types of glares.