(a) Use an amplitude-and-phase diagram to explain why a zone plate has secondary foci at distances of...

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(a) Use an amplitude-and-phase diagram to explain why a zone plate has secondary foci at distances of f/3, f/5, f/7 . . . .

(b) An opaque, perfectly circular disk of diameter D is placed perpendicular to an incoming plane wave. Show that, at distances r such that rF ≪ D, the disk casts a rather sharp shadow, but at the precise center of the shadow there must be a bright spot. How bright?

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