guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Double refraction
As the light waves enter the analyzer, they are reduced to their analyzer-parallel components,
and are thus subject to interference. The amount of retardation controls whether two related
waves obliterate each other completely, or whether they generate a resulting wave with an
amplitude from anything above zero to maximum height (Fig. 4-25). It is important here to
appreciate the conditions for constructive and destructive interference of originally orthogonal
waves which interfere in the analyzer (Figs. 4-24, 4-25) as opposed to the simple case
of interference of two waves vibrating in a single plane (cf. Fig. 4-1).
Considering interference of light waves in the N-S transmission direction of the analyzer, two
extreme cases can be distinguished:
Case A: If the retardation of the two waves corresponds to a phase shift of zero or wholenumber
multiples of λ, the condition of complete destructive interference is realized. The
analyzer-parallel wave components of this particular wavelength vibrate in opposite directions
and hence obliterate each other. No light is passing the analyzer (Fig. 4-25A).
Case B: If the retardation of the two waves corresponds to a phase shift of λ/2 or odd-number
multiples of λ/2, the condition of maximum constructive interference is realized. The
analyzer-parallel wave components of this particular wavelength vibrate parallel (“in phase”)
and thus are superimposed to form an interference wave of maximum amplitude (i.e.,
maximum light intensity). The light is completely transmitted by the analyzer (Fig. 4-25B), if
absorption effects of the polarizing filter are disregarded.
For any retardation between these extremes the intensity of the light transmitted by the
analyzer is reduced to a certain degree, depending on the exact phase shift (e.g., 50% for ¼ λ
and ¾ λ). If monochromatic light was used, mineral grains of variable orientation in a thin
section would show different levels of brightness, between black and maximum brightness, as
∆n depends on crystal orientation, and retardation is a function of ∆n (Γ = d*∆n; d = const).
Mineral grains with wedging-out edges would show a bright-and-dark-striped pattern corresponding
to a continuous variation in Γ relating to the change in d (∆n = const).
Crystal sections observed in white light under crossed polarizers appear in characteristic
interference colours that vary only in intensity as the microscope stage is turned, as long as
the mineral is in an off-extinction position (Fig. 4-26).
Raith, Raase & Reinhardt – February 2012
Figure 4-26. Interference colour of a forsterite crystal as the stage is rotated from the extinction
position into a 45° diagonal position. The interference colour does not change during
rotation, but its intensity does. In the crystal section parallel (100) birefringence is (n y –n x ) =
0.015 and retardation thus amounts to 25*10 3 nm * 0.015 = 375 nm.
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