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In physics, critical opalescence refers to the dramatic increase in scattering of light in the region of a continuous, or second-order, phase transition. Near the critical point, the properties of the liquid and gas phases become indistinguishable. The resulting density fluctuations are on such a large scale that they scatter visible light, giving the substance a cloudy or opalescent look. This phenomenon is an indicator of critical phenomena in fluids and can be observed in various materials under the right conditions.