Lecture 12: Optics II The first lecture on optics explores how to identify isotropic minerals from their refractive index and anisotropic minerals from their hirefringence and interference colors. This lecture discusses more powerful tools for identifying anisotropic minerals, including interference figures, optic signs, and 2V angles. Interference Figures Interference figures distinguish uniaxial and biaxial minerals and can be used to determine a mineral's optic sign. They are obtained by inserting the analyzer, flipping up the substage condenser, and rotating the Bertrand lens into place. The substage condenser causes the plane-polarized light from the illuminator to converge at the center of the sample, changing the illumination from orthoscopic to conoscopic. This step focuses the image on the plane near the top of the objective lens, and the Bertrand lens is required to refocus it. Bertrand lens Ocular Crystal plate Substage condeser Plane polarized light uminator Uniaxial Interference Figures There are two types of uniaxial interference figures: optic-axis figures and off-center optic-axis figures. Optic-axis figures occur when the c-axis is perpendicular to the stage. Isochromes Isogyres They consist of isochromes, isogyres, and a melatrope. As shown in the picture below, isochromes are concentric color bands formed from cones of light that diverge though the crystal and experience equal amounts of retardation. Note that the outer colors are the faintest because the outermost rays of light have the longest travel path through the crystal and experience the most retardation. This is demonstrated by comparing paths 1 and 4 in the first figure. Rays with the least Rays with the most retardation retardation Isochromes Sample Isogyres are formed where the polarization of the rays that make up the cone is perpendicular to the polarization of the analyzer. The melatrope represents the c-axis and is the point in the center where the two isogyres cross. 40 The 'o' ray vibrates tangentially to the isochromes The 'e' ray vibrates radially Close up of vibration directions in the isogyres Off-center interference figures have the same characteristics as the optic-axis figure. The have isochromes, isogyres, and a melatrope, but the melatrope is not in the center of the figure because the mineral's c-axis is inclined with respect to the stage. Off-center optic-axis figure Optic Sign Uniaxial minerals are designated positive or negative according to the relative magnitudes of their refractive indices. In a positive crystal, ε is larger than ω. In a negative crystal, ω is larger than ε. The sign of a crystal can be determined from its interference figure when a crystal plate is inserted. This plate is usually made out of mica or gypsum. The orientations of its fast and slow vibration directions are known, so viewing an interference figure through it causes the retardation of the light to change in a predictable way and shifts the interference colors.