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1.9 Interference figures

1.9 Interference figures - Interference figures Very...

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Unformatted text preview: Interference figures Very important tool to determine optical characteristics Uniaxial vs biaxial Optic sign 2V angle Technique Focus with highest power objective Be certain observation is conoscopic light Insert Betrand lens or remove ocular Flip in conoscope on some microscopes Most of them have to raise light sources as high as possible Interference figure forms on top of objective lens Betrand lens required to refocus the image Figure consist of isogyres and isochromes Nature of interference figure and patterns as stage rotated determines optical property Types of figures controlled by cut of the grain Isochromes: patterns of interference colors Isogyres: dark bands (extinction) Uniaxial Interference Figure Isogyres Isochromes Melatope Fig. 735 Uniaxial Interference Figure Three types: Optic axis figure Offcenter optic axis figure Flash Figure Optic Axis Figure Forms when optic axis perpendicular to stage Grain exhibits low interference color (extinct) Figure Black cross of isogyres Circular isochromes Middle of isogyres called melatope Location of optic axis Isochromes increase in order of color outward Origin of isochromes: Thicker and high birefringent minerals have more isochromes Light traveling along optic axis (melatope) has no retardation Light near melatope has low retardation (d and little higher Light far from melatope have higher retardation (d and increase more) E.g. calcite vs quartz Thin section Indicatrix Long path length, greater and d, higher interference colors Fig. 736 Origin of Isochromes and Isogyres rays vibrate tangent to isochromes rays vibrate perpendicular to isochromes Isogyres are where vibration directions are NS and EW, extinct Thin section Indicatrix Fig. 737 Offcenter OA figure Figure forms when OA is not perpendicular to stage Correct grain will have intermediate interference colors Use of figure similar to centered OA figure Offcenter OA figure If OA < ~30 to stage, melatope in field of view Isogyres swing around center of cross hairs If melatope is out of field of view, difficult but possible to determine optic sign Offcenter OA, melatope in field of view Optic Axis inside field of view Fig. 738 Offcenter OA figure, melatope outside field of view Thin section Indicatrix Optic Axis outside field of view Fig. 738 Optic Normal (Flash Figure) Formed when OA is parallel to stage Grains have highest interference colors Broad diffuse isogyres, split and leave field of view Not much use Determines orientation of OA e.g. pleochroism Fig.739 Determining Optic Sign Orientation of vibration directions known in each quadrant Insertion of accessory plate will cause subtraction and addition ' always points toward melatope, Orientation of vibration directions from fig. 337 Addition Subtraction > + < Fig. 7 40 Biaxial Interference Figures 5 major figures 2 useful ones: 3 worthless ones: Acute bisectrix (Bxa) figure Optic axis figure Obtuse Bisectrix (Bxo) figure Optic Normal figure (Flash figure) Offcenter figure Acute bisectrix figure Bxa axis (X or Z depending on sign) oriented perpendicular to stage Biaxial Indicatrix Optically positive Optically negative Fig. 727 Acute bisectrix figure Grains have intermediate to low interference colors (depends on 2V) Isogyres form cross that splits and leaves field of view as stage is rotated Two melatopes (i.e. two OA) Isochromes are oval or figure 8 around the melatope Acute bisectrix figure Optic Plane O p tic P e an l Grain at extinction Grain 45 from extinction Fig. 741 Optic Axis Figure Formed when OA is vertical These grains have zero or small retardation If 2V > 30, only one melatope (OA) in field of view If 2V very small, looks like an offcenter Bxa figure 2V < 30 2V > 30 Fig. 744 Determining Optic Sign Done with Bxa and OA figure Example of Bxa figure: Two light rays vibrate along Bxa axis (either Z or X) One has vibration of n , this one is perpendicular to the optic plane Other depends if mineral is + or If +, then vibration is n If , then vibration is n Use accessory plate to determine if vibration is fast or slow Addition Subtraction Fast on slow? Slow on slow? Shows if Bxa is Z or X Two vibration directions depend on which axis is Bxa Fig. 748 Light from bottom Determining optic sign with Biaxial OA figure Subtraction Addition Slow over fast subtraction Slow over slow addition Fig. 749 Determining 2V several techniques Bxa figure: Spacing between melatopes relates to 2V Depends on numerical apature (NA) of objectives Can guess within about 10 Numerical aperture 15 30 45 60 Fig. 751 Optic axis figure Curvature of the isogyre depends on 2V If 2V = 90, the isogyre is a straight line If 2V = 0, the isogyre forms a cross it is uniaxial Fig. 752 ...
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