A contact lens is an optical lens, located directly on the cornea of the eye. It is a corrective tool to modify and eliminate refractive defects of sight. Its function can be protective (preventive and therapeutic), diagnostic (i.e. during ophthalmological examinations) or cosmetic (i.e. to change eye colour).
The first contact lenses were made of glass. Current materials include silicone hydrogel. Contact lenses can be classified according to various criteria. The simplest classifications are hard lenses or soft lenses.