Two thick plates of uniaxial crystal, equally oriented and very little different in thickness, in the crossed nicols illuminate white light separately. Why can painting occur in the same environment, if you turn one plate relative to the other by 90º?
When the plates have the same orientation, the contributed path difference is big, i.e. corresponds to a high order of interference. In such conditions, the interference in white light, cannot be observed. If one of the plates is rotated with respect to another at 90 then the path difference will be the same, which is brought by a plate with a thickness equal to the thickness of the plates under consideration. With this difference in thickness the path difference can become so small that interference in white light will be possible.