40. (a) In a manner similar to that discussed in Sample Problem 36-5, we find the ratio should be d/a= 4. Our reasoning is, briefly, as follows: we let the location of the fourth bright fringe coincide with the first minimum of diffraction pattern, and then set sin θ= 4λ/d= λ/a(so d= 4a). (b) Any bright fringe which happens to be at the same location with a diffraction
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