Fire particles one at a time at a wall with two slits. Each arrives as a single dot — but where it can land is governed by a wave, |ψ₁+ψ₂|². Let thousands land and the dots build the famous interference fringes: single particles, wave statistics. Close a slit, or switch on a which-path detector, and the fringes vanish — because measuring which slit destroys the interference.
The fringe term here is N = 2 of the estate's one array factor — the very same function the Hall's diffraction grating reads at N ≫ 2. See both wings, one slit →
Click Fire one a few times — the dots look random. Hit Stream and watch the bands emerge from pure chance. Now Close one slit: the fringes collapse to a single broad blob (the slit's own diffraction envelope). Switch the which-path detector on: both slits are open, yet the fringes wash out — knowing the path erases the wave. The big number is Δy = λL/d; the self-test proves the pattern lands on it to ~1e−9, and that the dots really are drawn from |ψ|².