A quartz plate cut parallel to the optical axis is placed between the crossed nicols. The optical axis of the plate makes an angle of 45° with the main directions of the nicols. Calculate the minimum plate thickness at which one hydrogen line l1 = 6563 A will be greatly weakened, and the other l2 = 4102 A will have maximum intensity. The anisotropy value of quartz is Dn = 0.009.

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