Young's double-slit experiment (1801) demonstrated the wave nature of light. Two coherent slits separated by distance d produce an interference pattern with bright fringes at angles θ where d·sinθ = mλ. The intensity is I(θ) = I₀ cos²(πd sinθ/λ) · sinc²(πa sinθ/λ), where the first term gives interference fringes and the sinc² term is single-slit diffraction envelope. Adjust wavelength and slit separation to see how the pattern changes.