Interference colours are the colours observed while examining a mineral thin section in crossed nicols. These are colours.
Birefringence is the difference in the refractive indices of the O-ray and the E-ray. It is a number.
When anisotropic minerals are examined under the microscope between crossed nicols a number of interesting optical properties are observed.
Interference colours
Interference colours are the colours observed while examining a mineral thin section in crossed nicols.
To describe interference colours the colour and the order is used. for this a interference colour chart is used which is known as Michel-Levy chart
The interference colour depends on the following factors:
1. Retardation
2. Birefringence
3. Thickness of the thin section
4. Plane of the section being observed
Birefringence
When plane polarised light passes through an anisotropic mineral it splits into two rays which vibrate in directions perpendicular to each other. The two rays have different refractive indices. One is called the o-ray while the other one is called as the e-ray.
The difference between the refractive indices of the o-ray and the e-ray is called the Birefringence
The more the birefringence the higher will be the interference colour.