Three plane monochromatic waves with amplitudes 1, a and a (a 1) are incident on the plane z = 0, the first of them is normal to the plane, and the other two are at angles a and -a (figure). When the observation plane is shifted to the region z > 0, periodic changes in the contrast of the interference pattern are observed. Explain the phenomenon. What is the maximum and minimum contrast value? What are the positions of the observation plane in this case?

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