The milky appearance of certain transparent media when they are illuminated by visible polychromatic electromagnetic radiation. Opalescence occurs when particles are suspended in a solution. It also occurs in certain minerals, such as opal, in which there are thin surface films. Opalescence can occur in a system in which there are local fluctuations causing local fluctuations in the refractive index. Critical opalescence is opalescence that occurs at a critical point in various phase transitions, such as gas–liquid transitions due to density fluctuations.