Two coherent bundles of quasi-monochromatic unpolarized light of equal intensity cause interference fringes on the screen. The crystal plate of which thickness must be entered on the path of one of these bundles so that the interference fringes disappear and so that they could not be recovered by any glass plate, introduced into another beam? How will the picture change, if a polaroid is placed behind the crystal plate? In what position of the polaroid there will be no fringes?
Let’s divide a light wave into two components, the electric vectors of which are perpendicular and parallel to the principal axes of the plate. With the introduction of the plate the interference fringes from each component will be displaced. If the introduced plate is in the half-wave plate, then the difference of displacements will be half the width of the strip. In this case, the introduction of the plate, the interference fringes will disappear. With the introduction of the polaroid, they will appear again. An exception is the case when the axis of the polarizer is tilted at an angle of 45º to the axes of the plate. In this case, the interference fringes will not be observed.