兩個相消干涉的最小距離是什麼?急問!

Let the upper source be S1, and the lower source be S2
Let distance between S1 and A = L1, and distance between S2 and B = L2, distance between A and B be d

If destructive interference occurs at A, then we have
L1 - (d+ L2) = (n-1/2)入 ------------------ (1)

Point B is the next position where destructive interference occurs,
(L1 + d) - L2 = [(n+1) - 1/2]入 ------------------- (2)

(2) - (1): 2d = 入
i.e. d = 入/2