guide to thin section microscopy - Mineralogical Society of America

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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. 85
Guide to Thin Section Microscopy Double refraction Raith, Raase & Reinhardt – February 2012 Figure 4-25A. Schematic presentation of the behaviour of light passing through a birefringent crystal which is in a diagonal position. Polarizers are crossed. For simplicity, only monochromatic light is shown. Retardation is 1λ. For explanations see p. 88. 86
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