Light passes through a system of two crossed Nicol prisms between which a quartz plate cut at right angles to its optical axis is placed. Determine the minimum thickness of the plate which allows light of wavelength `436 nm` to be complately cut off by the system and transmits half the light of wavelength `497 nm`. The specific rotation constant of quartz for these wavelength is equal to `41.5` and `31.1` angualr degress per mm respectively.