The index of refraction (or refractive index) of an eyeglass lens material is a number that is a relative measure of how efficiently the material refracts (bends) light, which depends on how fast light travels through the material. Specifically, the refractive index of a lens material is the ratio of the speed of light in a vacuum, divided by the speed of light in the lens material. For example, the index of refraction of CR-39 plastic is 1.498, which mean light travels roughly 50 percent slower through CR-39 plastic than it does through a vacuum. The higher the refractive index of a material, the slower light moves through it, which results in greater bending (refracting) of the light rays. So the higher the refractive index of a lens material, the less lens material is required to bend light to the same degree as a lens with a lower refractive index. In other words, for a given eyeglass lens power, a lens made of a material with a high refractive index will be thinner than a lens made of a material with a lower refractive index. The refractive index of current eyeglass lens materials ranges from 1.498 (CR-39 plastic) to 1.74 (a specific variety of high-index plastic). So for the same prescription power and lens design, a lens made of CR-39 plastic will be the thickest lens available, and a 1.74 high-index plastic lens will be the thinnest.
The Abbe value (or Abbe number) of a lens material is an objective measure of how widely the lens disperses different wavelengths of light as light passes through it. Lens materials with a low Abbe value have high dispersion, which can cause noticeable chromatic aberration — an optical error visible as colored halos around objects, especially lights. When present, chromatic aberration is most noticeable when looking through the periphery of eyeglass lenses. It is least noticeable when looking directly through the central optical zone of the lenses. Abbe values of eyeglass lens materials range from a high of 59 (crown glass) to a low of 30 (polycarbonate). The lower the Abbe number, the more likely the lens material is to cause chromatic aberration. Abbe number is named after the German physicist Ernst Abbe (1840-1905), who defined this useful measure of optical quality.