A piece of glass, or other transparent substance, ground with two opposite regular surfaces, either both curved, or one curved and the other plane, and commonly used, either singly or combined, in optical instruments, for changing the direction of rays of light, and thus magnifying objects, or otherwise modifying vision. In practice, the curved surfaces are usually spherical, though rarely cylindrical, or of some other figure.

A transparent double convex (outward curve on both sides) structure between the iris and the vitreous humor. Two structures of the eye focus light onto the retina. The first is the cornea or front surface of the eye that provides about 65% of the focusing power of the eye. The human lens is located behind the iris and in front of the vitreous humor and provides the remaining focusing power for the eye. In younger patients (usually below age 45) the lens is able to adjust it's power allowing the eye to change it's focal length from distance to near.

Diffuser or refractor for a light fixture. Usually made of glass or plastic.

The portion of the eye that bends light rays and thus can focus an image on the retina.

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