guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Double refraction
Interference colours:
In contrast to monochromatic light, the use of white light provides a full spectrum of wavelengths
(spectral colours) which, for a given retardation, is modified in the analyzer through
interference such that certain wavelengths are transmitted at full intensity; others are reduced
to a variable degree or are obliterated entirely. White light exiting a colourless anisotropic
crystal comprises an infinite number of wave couples corresponding to all spectral colours,
each wavelength represented by a wave couple with mutually orthogonal vibration directions
that are fixed by the crystal's orientation (cf. Figs. 4-25A and B which show such single wave
couples). Interference in the analyzer between the two waves of each couple across the light
spectrum is a way of making retardation visible and also quantifiable. Any specific
retardation modifies the full white-light spectrum, generating a characteristic wavelength
spectrum and wave amplitude pattern, which in combination produce a unique interference
colour. Evidently, interference colours can only be generated from polychromatic light. They
correlate directly with retardation. The sequence of interference colours in relation to
increasing retardation is explained graphically in Figs. 4-27 and 4-28.
Raith, Raase & Reinhardt – February 2012
Figure 4-27. Constructive and destructive interference as a function of wavelength λ and
retardation Γ in the spectral range of visible light (see also Fig. 4-28). The variation of the
intensity of the light being transmitted by the analyzer is described by the equation I =
100[sin 2 (Γ/λ)180°].
To give a few examples: In the lower range of retardation (Γ = 0 ↔ 200 nm) black and grey
colour tones dominate as the complete colour spectrum of white light is affected by a
reduction of intensities. At around Γ = 250 nm only wavelengths from both ends of the
spectrum are slightly diminished which is perceived as close to white ("first-order white").
The range Γ = 400 ↔ 500 nm shows characteristic orange to light red interference colours, as
blue and green wavelengths are suppressed, while longer wavelengths dominate the spectrum.
This situation is reverse in the range Γ = 600 ↔ 650 nm. Here, the shorter wavelengths
dominate, which results in a blue interference colour. The distinctive purple colour at Γ = 551
nm ("first-order red") lies in a position where the intermediate wavelengths (green to orange)
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