Opalescence 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 of opalescent behaviour. One can still see through a slightly opalescent phase. The more particles and the bigger the particles are, the stronger the scattering arising from them and the cloudier the particular phase will look. At a certain concentration the scattering is so strong that all light passing through is scattered, so that it is not transparent any more.
Examples are the blue sky in the daytime and the yellowish-red sky at sunset. Another example can be made by adding a few droplets of milk to a glass of water: the milk looks bluish. If one looks through the milk at a light source, it looks yellowish-red.
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