Polychromatic or White Light consists of light of a variety of wavelengths, with the corresponding retardation the same for all wavelengths.
Due to different wavelengths, some reach the upper polar in phase and are cancelled, others are out of phase and are transmitted through the upper polar.
The combination of wavelengths which pass the upper polar produces the interference colours, which are dependant on the retardation between the fast and slow rays.
Examining the quartz wedge between crossed polars in polychromatic light produces a range of colours. This colour chart is referred to as the Michel Levy Chart and may be found as Plate I in Nesse.
At the thin edge of the wedge the thickness and retardation are ~ 0, all of the wavelengths of light are cancelled at the upper polarizer resulting in a black colour.
With increasing thickness, corresponding to increasing retardation, the interference colour changes from black to grey to white to yellow to red and then a repeating sequence of colours from blue to green to yellow to red. The colours get paler, more washed out with each repetition.
In the above image, the repeating sequence of colours changes from red to blue at retardations of 550, 1100, and 1650 nm. These boundaries separate the colour sequence into first, second and third order colours.
Above fourth order, retardation > 2200 nm, the colours are washed out and become creamy white.
The interference colour produced is dependant on the wavelengths of light which pass the upper polar and the wavelengths which are cancelled.
The birefringence for a mineral in a thin section can also be determined using the equation for retardation, which relates thickness and birefringence.
Retardation can be determined by examining the interference colour for the mineral and recording the wavelength of the retardation corresponding to that colour by reading it directly off the bottom of Plate I. The thickness of the thin section is ~ 30 µm. With this the birefringence for the mineral can be determined, using the equation:
See the example below.
This same technique can be used by the thin section technician when she makes a thin section. By looking at the interference colour she can judge the thickness of the thin section.
The recognition of the order of the interference colour displayed by a mineral comes with practice and familiarity with various minerals. In the labs you should become familar with recognizing interference colours.
Read Section on Page 46 in text on recognizing Interference Colour Order.