Critical opalescence

Critical opalescence is the dramatic scattering of light that turns a normally transparent fluid milky and white as it approaches its critical point — the temperature and pressure where the distinction between liquid and gas vanishes. The effect was the first direct visual evidence that thermal fluctuations can become macroscopically large.

Its cause is the divergence of the fluid's compressibility at the critical point. Since the magnitude of density fluctuations is governed by the compressibility, the fluctuations grow without bound, developing correlated regions of varying density that swell until they reach the wavelength of visible light. Such regions scatter light strongly and over all directions, producing the characteristic cloudy glow.

Critical opalescence is the everyday face of critical phenomena. The way the correlation length and other quantities diverge near the critical point is described by critical exponents that are remarkably universal — the same for fluids, magnets and alloys — a universality explained only decades later by the renormalisation group.

Observed in the nineteenth century and explained by Marian Smoluchowski (1908) and Albert Einstein (1910) in terms of diverging density fluctuations, providing further strong evidence for the molecular theory of matter and foreshadowing the modern study of critical phenomena.