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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
- IIT JAM
- Geology (GG)
Q-if fast ray vibrating along the length of tetragonal and hexagonal crystals, then the crystals will be ? a- biaxial + b- biaxial c-uniaxial+ d-uniaxial
Q-If Fast ray vibrating along the length of Tetragonal and Hexagonal Crystals, then the Crystals will be ? a- Biaxial + b- Biaxial c-Uniaxial+ d-Uniaxial
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Saurabh 1![best-answer]()
Correct option most probably will be (D). The optic sign of the crystal in given orientation is negative (Uniaxial -ve) . I've attached figure for better understanding and to have some good idea about the optic nature and it's related norms. Things to note down are-- 1) If the given crystal in any specific orientation is uniaxial negative, the fast ray of the accessory plate will increase the retardation (Or simply addition) in the direction of inserting of accessory plate. 2) Suppose we take length fast gypsum plate (what is length fast, length slow, positive and negative elongation? -- Don't worry you can get it from any mineralogy book like Dexter Perkins, WD Nesse and Paul F Kerr), so as we are much aware of this fact that we usually deal with blue and yellow color in case of interference figures. Taking this into consideration, as we insert length fast Gypsum plate, blue interference color is being observed in quadrant (II) and (IV) respectively, which surely indicates the addition . So addition of colors, in turn increase in retardation and clearly fast ray of the accessory plate is parallel to the fast ray of the given crystals(along length). So this will result in uniaxial negative.