Critical opalescence

In physics, critical opalescence refers to the dramatic increase in scattering of light in the region of a continuous, or second-order, phase transition.

The resulting density fluctuations are on such a large scale that they scatter visible light, giving the substance a cloudy or opalescent look.

Originally reported by French physicist Charles Cagniard de la Tour in 1823 in mixtures of alcohol and water, its importance was recognised by Irish chemist Thomas Andrews in 1869 following his experiments on the liquid-gas transition in carbon dioxide; many other examples have been discovered since.

In 1908 the Polish physicist Marian Smoluchowski became the first to ascribe the phenomenon of critical opalescence to large density fluctuations.

As critical point is approached, density difference decreases along with the interfacial surface tension, so that precipitation takes place as increasingly fine and refractory to settling emulsion.