Refraction index of the lenses
The refraction index is the capacity of an optical material to bend light. The denser the material is with the light, the slower it travels through it and the higher the index.
This fact is important for the lenses of the glasses, since the higher the index of refraction, thinner and aesthetic will be. The usual values of the refractive index are: normal (1.50), half (1.60) and high (18.104.22.168.1.74, 1.80 and 1.90)