opalescence

from Wikipedia, the free encyclopedia
Black precious opal with a full, opalescent play of colors
Opalescent milk opal in the host rock (matrix)

As opalescence , the color is referred to some substances by the scattering and possibly interference caused the light to small structures in the fabric. The structures causing the scattering are smaller than the wavelength of the light. This differentiates this effect from dichroism , which describes a two-color in homogeneous crystals. Opalescence occurs in many cloudy media, which, however, changes into non-colored opacity depending on the particle size .

opal

In the namesake Opal , these structures are formed by tiny spheres of hydrated silica gel , which, when arranged regularly, show a lively, multicolored play of colors through the interference and only such opals are referred to as "opalescent". A similar effect occurs with ammolites and mother-of-pearl , the cause of which, however, lies in the layered structure of the material.

In the case of “common opal” (e.g. “milk opal”), which only has a milky bluish to pearly sheen, the term “opalescence” (verb: opalescent ) is used to distinguish it.

A slight opalescence in colloidal silicon dioxide (hydrodynamic diameter: 92.7 nm).

Cloudiness

Mostly, however, the term is used to describe a particular turbidity in colloidal dispersions . The dispersed particles are disordered here, so that no interference occurs as with opal. Instead, the color results from the scattering: Since the particles are smaller than the wavelength of the light, the scattering mechanism is the wavelength-dependent Rayleigh scattering . The scattered light therefore contains a higher proportion of blue, and the transmitted light accordingly has a higher proportion of red. A well-known example of this is the blue color of the sky and the reddish sun as it sets.

Transition to opacity

However, there is also a smooth transition to opacity : If the particles become larger than the wavelength, Rayleigh scattering is replaced by Mie scattering , which is independent of the wavelength . This then leads to the fact that the scattered light also contains more red components, thus becoming whiter. Overall, the spread also becomes weaker. The color becomes less and less noticeable, with pure opacity the scattered light is white. There is also an example of this in the color of the sky: If there is a lot of haze in the atmosphere, Mie scattering causes additional white components to enter the scattered light. This makes the blue sky color weaker and whiter. The same can be observed near the horizon, which shows a much lighter blue. Because of this, the clouds are also white.

Examples

  • An experiment on this is easy to carry out: a few drops of milk are placed in a glass of water so that the water becomes cloudy. The milky water appears bluish in incident light and reddish in transmitted light. The Tyndall effect can also be demonstrated with a small light source .
  • Opalescent media are frosted glass , tooth enamel , smoke
  • Opalescence also occurs in the air. The blue light is scattered by the smallest air molecules (oxygen and nitrogen molecules) and the red light is allowed through. That is why the sky and distant mountains are blue. With higher humidity, all colors are scattered and the blue appears white-brightened.
  • When diluting aniseed schnapps, the louche effect occurs, a spontaneous separation of the alcohol-oil phase, which leads to small aniseed oil droplets in the water, which show the opalescence.
  • Flop paintwork , for example on vehicles or mobile phones

Critical opalescence

Critical opalescence is called a phenomenon that occurs in fluids near and at the critical point due to density fluctuations. Parts of the fluid constantly change back and forth between the liquid and gaseous state. The generated local fluctuations in density are in the order of magnitude of the correlation length , i.e. usually the mean molecular distance. If one approaches the critical point, the correlation length increases considerably. The critical opalescence occurs exactly when the correlation length comes into the size of the wavelength of the light - because then the light is strongly scattered in these areas. If the area of ​​critical opalescence is reached, the short-wave light is therefore first scattered; directly at the critical point, however, the entire spectrum of visible light and the fluid appears milky.

literature

  • Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 54 .
  • Archie Kalokerinos: Opal - gemstone of a thousand colors . Kosmos Society for Friends of Nature, Franckh'sche Verlagshandlung, Stuttgart 1981, ISBN 3-440-05021-1 , p. 62 ff .

Individual evidence

  1. ↑ Marcel Minnaert: Light and color in nature . 1st edition. Birkhäuser Verlag, Basel, Boston, Berlin 1992, ISBN 3-7643-2496-1 , pp. 321 and 322 .