In optics the refractive index (or index of refraction) n of a substance (optical medium) is a number that describes how light, or any other radiation, propagates through that medium.

Refractive index of materials varies with the wavelength. This is called dispersion; it causes the splitting of white light in prisms and rainbows, and chromatic aberration in lenses. Inopaque media, the refractive index is a complex number: while the real part describes refraction, the imaginary part accounts for absorption.

The concept of refractive index is widely used within the full electromagnetic spectrum, from x-rays to radio waves. It can also be used with wave phenomena other than light (e.g.,sound). In this case the speed of sound is used instead of that of light and a reference medium other than vacuum must be chosen.

For infrared light refractive indices can be considerably higher. Germanium is transparent in a wavelength of 589 nanometers and has a refractive index of about 4, making it an important material for infrared optics.

SiC refraction index: 2.55 (infrared; all polytypes)