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The classic physical optics textbook approach to double-refraction starts from Huyghens constructions of wave fronts and from the optical indicatrix. Optical indicatrices are useful for a systematic description of optical properties in crystals, but students do not usually consider them an easy subject, and, therefore, shy away from optical crystallography. This is unfortunate since a basic understanding of optical crystallography is prerequisite to a correct interpretation of phenomena observed with the polarizing microscope, the most commonly used tool for the detailed study of rocks. Generally, students are comfortable with simple optical terms like reflection and refraction, while it is uncommon that they actually have seen double-refraction and noticed that crystals polarize light. Many have an unnecessarily complicated idea about vibration directions, interference colors, and interference figures; they assume such phenomena always require a microscope to observe. This is not so. Students well trained in thin section microscopy are often surprised that interference figures can be made visible macroscopically. The purpose of the experiments below is to impart an intuitive understanding of the interaction between light and crystals and, thus, of optical crystallography. This will help to demystify what is seen in the polarizing microscope, and will better prepare the student for the introduction of optical indicatrices as 3-D models to describe the directional dependence of light velocities, and thus refractive indices in anisotropic crystals.
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