By placing a plate cut from Iceland spar parallel to its optical axis between crossed nicols, it is possible to create a monochromator that allows, for example, to delay one of the lines of the sodium doublet and allow the other to pass through. Find what the minimum thickness d of the plate should be and how it should be oriented. The refractive indices of Iceland spar for the line l1 = 589.0 nm are ne1 = 1.48654 and n01 = 1.65846, for the line l2 = 589.6 nm they are ne2 = 1.48652 and n02 = 1.65843.

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