Two coherent beams of quasi-monochromatic unpolarized light of equal intensity produce interference fringes on the screen. What thickness of crystal plate must be introduced into the path of one of these beams so that the interference fringes disappear and, moreover, in such a way that they cannot be restored by any glass plate introduced into another beam? How will the picture change if a Polaroid is placed behind the crystal plate? At what position of the Polaroid will there be no interference fringes?

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