Tyndall effect

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Fog penetrated by sunlight, limited by tree shade
Laser beam in vapor particles
The effect only occurs on the sunny side, so there seems to be a shadow to the left of the Watzmann .

The Tyndall effect [ tɪndl- ] describes the scattering of light at microscopically small suspended particles with dimensions similar to the wavelength of light, generally, in a "turbid medium", particularly in a liquid or a gas suspended are. The effect is named after its discoverer John Tyndall , who studied the scattering of light in colloidal solutions. A tyndalloscope or a nephelometer is used as a measuring device .

principle

Due to the Mie scattering of the light, bundles of rays are scattered out sideways . This makes the entire light beam visible from the side. For example, when the sun is shining in the haze or fog , you can see so-called bundles of rays , or at night you see the light cones (Tyndall cones) from headlights in fog or clouds .

Since the effect increases in intensity with decreasing wavelength , the scattered light is often more bluish in color than the primary light passing through. However, the effect itself is not responsible for the blue color of the sky. John William Strutt, 3rd Baron Rayleigh , was the first to postulate in 1899 that no colloidal particles are needed for the blue of the sky, but that pure air also causes the corresponding scattering, the Rayleigh scattering named after him . It was not until 1918 that his son Robert Strutt, 4th Baron Rayleigh (1875–1947) succeeded in proving the sky blue in an experiment.

Applications

Optical smoke detectors use the Tyndall effect by the presence of smoke - particles of light from a light beam out on a light-sensitive sensor is scattered. With clean air, there is no scattering and the sensor can therefore not detect any scattered light. An alarm is triggered as soon as the sensor signal exceeds a defined threshold value .

In ophthalmology , this scattering effect is used for differential diagnostics: If one observes the scattering of the slit lamp light used for the examination inside the eye, this represents a positive Tyndall effect . It indicates particles floating in the aqueous humor , for example proteins can accumulate there as a result of a inflammatory process of the adjacent tissue structures.

Tyndall used the effect named after him to detect and quantify air pollution in London in 1871 .

When checking solutions , the Tyndall effect provides information that a substance is not completely dissolved.

See also

Individual evidence

  1. ^ [1] Academic dictionaries and encyclopedias
  2. ↑ [2] "Scattering" - University of Hanover
  3. ^ Carsten Möhlmann: Dust measurement technology - then until now. In: Hazardous substances - cleanliness. Air . 65, No. 5, 2005, ISSN  0949-8036 , pp. 191-194.
  4. ↑ DIN ISO 14887: 2010-03 sample preparation; Process for dispersing powders in liquids (ISO 14887: 2000). Beuth Verlag, Berlin, p. 22.