Moiré

Two layers of fine slits lie on a light box, and light gets through only where both are open. Turn the top layer a little, or change its slit spacing slightly, and broad bands appear; the smaller the mismatch, the further apart they are, 2π / |k₁ − k₂|, where the pair of ice-blue points in the corner spectrum are ±(k₁ − k₂). Press play beside Top-layer shift: the top layer moves by one slit spacing, and the bands move by one band spacing.