Eyeglasses and contact lenses are devices that correct refractive errors in vision. Eyeglass lenses are mounted in frames that are worn on the face, sitting mostly on the ears and nose, so that the lenses are positioned in front of the eyes.

Lenses work by changing the direction of light so that images come into focus on the retina. The greater the index of refraction of the lens material and the greater the difference in the curvature between the two surfaces of the lens, the greater the change in direction of light that passes through it and the greater the correction.