The ratio of the speed of light in air to its speed its speed in a substance. The ratio indicates the optical density of a substance. Measurement of the refractive index with the aid of a refractometer helps a gemologist to identify a gemstone. In gemology this is done with the use of yellow (sodium light). The more the gemstone bends the light, the higher the refractive index. The refractive index is one of the determinative optical properties of gemstones.