What are indices of refraction and birefringence

Refraction and birefringence

If we take the split thrombohedron of a calcite and place it on a newspaper, the writing appears twice. This phenomenon is called birefringence. For a more precise understanding, the concept of light refraction should first be explained: If you put a stick in a container with water, the stick appears kinked. This phenomenon can be explained by a special property of light: light rays that penetrate transparent materials change their direction when they enter the interface. If you direct a bundled light beam into water at an oblique angle, it will be bent at the transition between air and water.

The angle of refraction β can be calculated if the entrance angle α and the Refractive indices n1 and n2 of both media are known. Due to the phenomenon of refraction objects in the water appear closer to the surface than they actually are. Most of the time, the water depth of crystal clear waters is underestimated. If you hold a drinking straw in water, it looks from above as if the drinking straw is snapping off:

A drinking straw is submerged in water.

Transparent minerals that crystallize according to the cubic crystal system show the same effect of simple light refraction. All other minerals and also the calcite are birefringent. The ray of light is split into two rays: the ordinary or ordinary ray o, which is unbroken when the light enters straight, and the extraordinary or extraordinary ray e, which is deflected when the light enters straight.


Example for calcite at the wavelength λ = 589.3 nm

The refractive index for the ordinary ray is nO = 1,6584.
The index of refraction for the extraordinary ray is ne= 1,4864.

The strength of the birefringence results from the difference ne - nO:
1.4864 - 1.6584 = −0.1720 (optically negative)

The two rays pass through the crystal at different speeds. With the help of an optical arrangement, the two light beams can be superimposed, so that the phenomenon of interference occurs. As a result, different colored rings can be seen. The phenomenon can be used to determine minerals. Geologists take thin slices of a mineral or rock and examine it in polarized light. Based on the colors that appear, you can determine the mineral or the components in the rock.