Eyeglass lenses are often categorized by their refractive index (also called index of refraction). The refractive index of any substance is the speed of light in that substance compared to the speed of light in a vacuum expressed as a ratio. Light travels fastest in a vacuum and slower through different materials.

Refractive Index = speed of light in a vacuum/speed of light in the comparative material The refractive index of a basic plastic lens (CR-39) is 1.498 meaning that light travels 1.498 times faster in a vacuum than it does through the plastic lens.

The higher the index of refraction the thinner a lens can be to get the same refraction effect. Optical lenses are now classified into the following categories:

· Normal Index: 1.48-1.54

· Mid Index: 1.54-1.64

· High Index: 1.64-1.74

· Ultra High Index: 1.74 and above.