SUPERCRITICAL FLUID & CRITICAL OPALESCENCE

Near the critical point (Tc, Pc), density fluctuations grow to macroscopic scale. Correlation length ξ ~ |T−Tc|^(−ν) diverges (ν ≈ 0.63 for 3D Ising universality). These large fluctuations scatter light of all wavelengths — critical opalescence. Compressibility κ_T ~ |T−Tc|^(−γ) diverges (γ ≈ 1.24). The van der Waals equation: (P + a/V²)(V − b) = RT gives a critical point at Tc = 8a/(27Rb), Pc = a/(27b²).