You hear these numbers a lot. What do they mean? Students with high myopia often hear such a sentence: "Your degree is a little high, do you need to upgrade the lens to 1.67?" Otherwise the eyeglass online will come out too thick."   The above mentioned 1.67 refers to the refractive index of the lens. At present, the refractive index of the lens is mainly divided into 1.56, 1.61,...