When observing phase (transparent) structures using the dark field method in the common focal plane of lenses L1 and L2 (figure), a wire P is installed on the optical axis. Estimate its permissible diameter (dmax and dmin) for observing on the screen E the interference pattern from a phase sinusoidal grating with a period = 2 mm, illuminated by a normally incident plane wave of length l = 0.5 µm. Lens diameter L2 is D = 2 cm, focal length f = 20 cm.

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