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Interference (YDSE): uniform fringe width β = λD/d, all maxima equally bright ($4I_{0}$), perfectly dark minima (equal sources), condition d sinθ = nλ for bright. Diffraction (single slit): non-uniform pattern, central maximum twice as wide as secondaries, rapidly decreasing intensity for higher orders, condition a sinθ = nλ for dark (opposite to interference). Polarization: proves transverse nature, Malus's law I =$I_{0}$c$os^{2}$θ, Brewster's angle tan i_B = n. Key distinctions: interference requires two coherent sources while diffraction occurs with one; interference maxima are equally bright while diffraction maxima diminish; the nλ condition gives maxima in interference but minima in diffraction. In combined double-slit patterns, the interference fringes are modulated by the single-slit diffraction envelope. The practical difference is that interference involves superposition of a few waves while diffraction involves continuous superposition across an aperture.$