One of the methods for observing phase (transparent) objects is that the observation plane P is shifted by a certain distance l relative to the plane P0 conjugate with the object (i.e., the plane in which, in accordance with geometric optics, its image is located, drawing ). In this case, the contrast of the observed picture periodically changes as l changes. Find the period d of the phase sinusoidal grating if, in the circuit shown in the figure, its contrast image is in the first

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