guide to thin section microscopy - Mineralogical Society of America

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Guide to Thin Section Microscopy Double refraction Raith, Raase & Reinhardt – February 2012 Figure 4-25B. 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 ½ λ. For explanations see p. 88. 87
Guide to Thin Section Microscopy Double refraction Explanations for Figures 4-25A and 4-25B Case A: Maximum destructive interference (0% transmission by the analyzer) if the retardation accumulated in the crystal is 1λ. The same applies if the retardation is zero or a whole-number multiple of λ [Γ = n*λ; n = 0,1,2,3,…]. A light wave emitted from a light source experiences the following modifications (3-D model): E-W polarization in the lower polarizer; splitting up into two light waves with mutually perpendicular vibration directions as the light enters a birefringent crystal. Inside the crystal the two waves travel at different velocity and have different wavelengths. As the two waves exit the crystal they maintain their vibration directions but revert back to the wavelength and velocity of light below the crystal. From the point of exit the retardation accumulated within the crystal remains constant. The analyzer-parallel (N-S) components of the two waves entering the analyzer vibrate in opposite directions and thus eliminate each other by interference. Case B: Maximum constructive interference (transmission by the analyzer at maximum amplitude) if the retardation accumulated in the crystal is λ/2. The same applies if the retardation is an odd-number multiple of λ/2 [Γ = (2n+1)*λ/2; n = 0,1,2,3,…]. A light wave emitted from a light source experiences the following modifications (3-D model): E-W polarization in the lower polarizer; splitting up into two light waves with mutually perpendicular vibration directions as the light enters a birefringent crystal. Inside the crystal the two waves travel at different velocity and have different wavelengths. As the two waves exit the crystal they maintain their vibration directions but revert back to the wavelength and velocity of light below the crystal. From the point of exit the retardation accumulated within the crystal remains constant. The analyzer-parallel (N-S) components of the two waves entering the analyzer vibrate in parallel arrangement and thus produce a resulting wave of maximum amplitude. Raith, Raase & Reinhardt – February 2012 Green: Plane-polarized light exiting the polarizer. Red: fast wave inside the crystal (correlates with n x ') and its corresponding wave outside the crystal; blue: slow wave inside the crystal (correlates with n z ') and its corresponding wave outside the crystal. The left-hand side of Figs. A and B (column 1) displays schematic 3-D presentations of the crystal, the polarizers and the light waves. The vector decomposition of the original light wave as it enters the crystal as well as the vector relationships in the analyzer are shown in a view from the top (i.e., in the direction of the microscope axis; column 2). The same viewing direction applies to the vibration directions at the various positions of light transmission in the microscope (column 4). Column 3 represents a projection of the light waves into an E-W plane containing the microscope axis. It should be noted that the red and blue light waves vibrate at 45° to the drawing plane and perpendicular to each other (as seen in the 3-D models). The light exiting the analyzer in the case of constructive interference (case B) vibrates N-S und hence perpendicular to the projection plane. 88
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