Critical opalescence is a phenomenon which arises in the region of a continuous, or second-order,
phase transitionA phase transition is a natural physical process. It has the characteristic of taking a given medium with given properties and transforming some or all of that medium, into a new medium with new properties. Phase transitions occur frequently and are found everywhere in the natural world...
. Originally reported by
Thomas AndrewsThomas Andrews FRS was a chemist and physicist who did important work on phase transitions between gases and liquids.-Life:...
in 1869 for the liquid-gas transition in
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
, many other examples have been discovered since. The phenomenon is most commonly demonstrated in binary fluid mixtures, such as methanol and cyclohexane. As the critical point is approached the sizes of the gas and liquid region begin to fluctuate over increasingly large length scales.
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Critical opalescence is a phenomenon which arises in the region of a continuous, or second-order,
phase transitionA phase transition is a natural physical process. It has the characteristic of taking a given medium with given properties and transforming some or all of that medium, into a new medium with new properties. Phase transitions occur frequently and are found everywhere in the natural world...
. Originally reported by
Thomas AndrewsThomas Andrews FRS was a chemist and physicist who did important work on phase transitions between gases and liquids.-Life:...
in 1869 for the liquid-gas transition in
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
, many other examples have been discovered since. The phenomenon is most commonly demonstrated in binary fluid mixtures, such as methanol and cyclohexane. As the critical point is approached the sizes of the gas and liquid region begin to fluctuate over increasingly large length scales. As the density fluctuations become of a size comparable to the wavelength of light, the light is scattered and causes the normally transparent fluid to appear cloudy. Tellingly, the
opalescenceOpalescence is a type of dichroism seen in highly dispersed systems with little opacity. The material appears yellowish-red in transmitted light and blue in the scattered light perpendicular to the transmitted light. The phenomenon is named after the appearance of opals.There are different degrees...
does not diminish as one gets closer to the critical point, where the largest fluctuations can reach even centimetre proportions, confirming the physical relevance of smaller fluctuations.
In 1908 the Polish physicist
Marian SmoluchowskiMarian Smoluchowski was a Polish scientist, pioneer of statistical physics and a mountaineer.-Life:...
became the first to ascribe the phenomenon of critical opalescence to large density fluctuations. In 1920
Albert EinsteinAlbert Einstein was a theoretical physicist. His many contributions to physics include the special and general theories of relativity, the founding of relativistic cosmology, the first post-Newtonian expansion, explaining the perihelion advance of Mercury, prediction of the deflection of...
showed that the link between critical opalescence and
Rayleigh scatteringRayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light, which may be individual atoms or molecules. It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases...
is quantitative.
External links
More-detailed experimental demonstrations of critical opalescence may be found at
- http://physicsofmatter.com/NotTheBook/CriticalOpal/Explanation.html
- http://www.ucl.ac.uk/~uccaata/work/opalescence/opalescence.html
- http://www.msm.cam.ac.uk/doitpoms/tlplib/solid-solutions/demo.php