Britannica Sources used in this answer

Why do uniaxial crystals produce two refractive indices?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A rough, translucent yellow crystal of elemental sulfur displayed against a solid black background. — crystal A crystal form of elemental sulfur. Sulfur has an atomic weight of about 32.06 atomic mass units (amu), which means that its molar mass is 32.06 grams.

Uniaxial crystals produce two refractive indices because they are anisotropic media that exhibit double refraction, a phenomenon where a single ray of unpolarized light is split into two polarized components vibrating in mutually perpendicular planes.12

These two components are known as the ordinary ray and the extraordinary ray. The difference in refractive indices arises from the crystal's internal structure:1
* The ordinary ray experiences a constant refractive index in all directions.1
* The extraordinary ray has a refractive index that varies depending on the direction of travel relative to the crystal's optical axis.1

Specifically, uniaxial crystals—which include the tetragonal, hexagonal, and rhombohedral systems—yield one refractive index for light vibrating parallel to the optical axis and another for light vibrating perpendicular to it. Because the speed of light within the medium depends on its refractive index, these two rays travel at different speeds, causing the light to bend at different angles. This distinction vanishes only when light travels directly along the crystal's optic axis.312