A quartz plate cut parallel to its optical axis is placed between two crossed Nicol prisms so that their principle directions form an angle of 45° with the optical axis of the plate. What is the minimum thickness of that plate transmitting light of wavelength λ1 = 643 nm with maximum intensity while greatly reducing the intensity of transmitting light of wavelength λ2 = 564 rim? The difference of refractive indices for extraordinary and ordinary rays is assumed to be equal to ne – no = 0.0090 for both wavelengths.