Diopter is the unit of the refractive power of a lens or optical system corresponding to the refractive power of an optical glass with a focal length of 1 m. It is denoted by the letter D. The glass force is inversely proportional to the focal length, i.e. the longer the focal length, the weaker the glass. For example, a 10.0 D glass has a focal length of 10 cm, and a 2.0 D glass is 50 cm. A plus sign (+) with D indicates that the lens is a collective lens, a minus sign (-) indicates that the lens is diffusing.
The refractive power of the glass is determined using a diopter (see Points).