Log1989 problem number 21.14

Log1989 problem number 21.14

At point A there is a source of monochromatic light (500nm). Diaphragm D with a hole of radius r=1mm moves from a point a₁=1 m away from point A to a point a₂=1.75m away from A. How many times N will darkening be observed at point B if AB=2m?

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