index of refraction

When an incident light enters a medium, it will travel at a slower velocity compared to the original speed of light. ¹ The ratio of this speed reduction is called the index of refraction (IOR), which is commonly denoted as (pronounced as “eta”), where

where is the speed of light in vacuum, and is the speed of light in the medium. For example, a material with an IOR of 2 propagates light at half of the speed of light.

When encountering boundaries with a sudden change in the refraction index, the light wave also undergoes significant reflection. For example, an air-diamond surface (IOR 2.42) is more reflective compared to an air-glass surface (IOR 1.5). In this sense, the IOR provides an explanation of the visual appearance of objects around us. ¹

Wavelength-Dependent IOR

Many materials have a wavelength-dependent refractive index, which causes them to refract light in different directions depending on different wavelengths. The phenomenon is called dispersion.

See Also

• Snell’s Law - describes the direction where light bends using IOR
• Fresnel equations - answers how much light reflects and refracts

Footnotes

1. PBRT v4 9.3.2 The Index of Refraction ↩ ↩²