Opalescence

Opalescence, the color of certain materials is known, which is caused by the scattering and interference of light may be of small structures in the material. The scattering inducing structures are smaller than the wavelength of light. To prevent this effect from the dichroism, which describes a two-tone in homogeneous crystals is different. In many turbid media opalescence occurs, but is dependent on the particle size in the non-colored opacity.

Opal

When named Opal these structures are formed by tiny spheres of hydrated silica, the show at regular arrangement a lively, buntfleckiges color play by the interference and only those opals are referred to as " opalescent ". A similar effect also occurs when Ammoliten and pearl, the cause, however, is located in the layered structure of the material.

In the " commons Opal" (eg " milk opal "), which shows only a milky bluish to pearly luster, is used to distinguish the term " opalescence " (verb: opalescent ) is used.

Cloudiness

Most often, the term is also used to describe a particular turbidity in colloidal dispersions. The dispersed particles are disordered here, so that interference will not occur as in the Opal. The color results from the dispersion instead Because the particles are smaller than the wavelength of light, the scattering mechanism is the wavelength-dependent Rayleigh scattering. Thus, the scattered light contains more blue, the transmitted light accordingly a higher proportion of red. A well-known example is the blue color of the sky and the reddish sun at its setting.

Transition to opacity

However, there is also a smooth transition to opacity: If the particles are larger than the wavelength, the Rayleigh scattering is replaced by the independent of the wavelength Mie scattering. This then results in that the scattered light also contains more red component, thus, off-white. Overall, the scattering is weaker. Thus, the color makes less and less noticeable with pure opacity, the scattered light is white. To this end, there is also an example in the color of the sky: If a lot of haze in the atmosphere, coming by the Mie scattering more white in the scattered light. Thus, the blue sky color is weaker and whitish. The same can be observed in the vicinity of the horizon, which is a significantly lighter blue. Even the clouds are so white.

Examples

• An experiment can be carried out to be simple: in a glass of water a few drops of milk are given, so that the water is cloudy. The milky water appears blue in reflected light, transmitted light reddish. With a small light source and the Tyndall effect can be demonstrated.
• Opalescent media are frosted glass, enamel, smoke
• When diluting Anisschnäpsen the louche effect occurs, a spontaneous separation of the alcohol -oil phase, which leads to small anise oil droplets in the water, showing the opalescence.
• Flop paint, for example, in vehicles or mobile phones

Critical opalescence

Critical opalescence is called a phenomenon that occurs in the fluids in the proximity and at the critical point due to density fluctuations. Parts of the fluid switch constantly between liquid and gaseous states back and forth. Local variations in the density of the produced moving in the order of the average distance between molecules, that is in the correlation length. As one approaches the critical point, the correlation length increases considerably. The critical opalescence occurs if and when the correlation length is in the size of the wavelength of the light - because then the light on these areas can be scattered. When you reach the area of critical opalescence, the short-wavelength light is scattered ergo first; directly at the critical point, the entire spectrum of visible light is scattered and the fluid appears milky.