Multi-slit interference: I(θ) = sinc²(πa sinθ/λ) × |sin(Nπd sinθ/λ)/sin(πd sinθ/λ)|². Single-slit envelope (diffraction) modulates the multi-slit interference pattern. Coherence length L_c introduces a Gaussian envelope that washes out fringes at large path differences. "Animate λ" sweeps wavelength to show fringe dispersion.