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Tyndall effect
The scattering of light by particles in a colloid or a fine suspension

The Tyndall effect is the scattering of light by particles in a colloid or fine suspension, causing shorter wavelengths, like blue light, to scatter more strongly than longer wavelengths due to an inverse fourth power relationship with wavelength. This effect explains the blue color of tobacco smoke and the smoke from two-stroke motorcycles. The phenomenon occurs when particles sized between 40 to 900 nm scatter light within the visible spectrum. It is widely used in nephelometers to study aerosols, and its investigation led to inventions like the ultramicroscope. The effect is named after physicist John Tyndall.

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History

Prior to his discovery of the phenomenon, Tyndall was primarily known for his work on the absorption and emission of radiant heat on a molecular level. In his investigations in that area, it had become necessary to use air from which all traces of floating dust and other particulates had been removed, and the best way to detect these particulates was to bathe the air in intense light.5 In the 1860s, Tyndall did a number of experiments with light, shining beams through various gases and liquids and recording the results. In doing so, Tyndall discovered that when gradually filling the tube with smoke and then shining a beam of light through it, the beam appeared to be blue from the sides of the tube but red from the far end.6 This observation enabled Tyndall to first propose the phenomenon which would later bear his name.

In 1902, the ultramicroscope was developed by Richard Adolf Zsigmondy (1865–1929) and Henry Siedentopf (1872–1940), working for Carl Zeiss AG. Curiosity about the Tyndall effect led them to apply bright sunlight for illumination and they were able to determine the size of 4 nm small gold nanoparticles that generate the cranberry glass colour. This work led directly to Zsigmondy's Nobel Prize for chemistry.78

Comparison with Rayleigh scattering

Rayleigh scattering is defined by a mathematical formula that requires the light-scattering particles to be far smaller than the wavelength of the light.9 For a dispersion of particles to qualify for the Rayleigh formula, the particle sizes need to be below roughly 40 nanometres (for visible light), and the particles may be individual molecules.10 Colloidal particles are bigger and are in the rough vicinity of the size of a wavelength of light. Tyndall scattering, i.e. colloidal particle scattering,11 is much more intense than Rayleigh scattering due to the bigger particle sizes involved. The importance of the particle size factor for intensity can be seen in the large exponent it has in the mathematical statement of the intensity of Rayleigh scattering. If the colloid particles are spheroid, Tyndall scattering can be mathematically analyzed in terms of Mie theory, which admits particle sizes in the rough vicinity of the wavelength of light.12 Light scattering by particles of complex shape are described by the T-matrix method.13

Blue irises

The color of blue eyes is due to the Tyndall scattering of light by a translucent layer of turbid media in the iris containing numerous small particles of about 0.6 micrometers in diameter. These particles are finely suspended within the fibrovascular structure of the stroma or front layer of the iris.14 Some brown irises have the same layer, except with more melanin in it. Moderate amounts of melanin make hazel, dark blue and green eyes.

In eyes that contain both particles and melanin, melanin absorbs light. In the absence of melanin, the layer is translucent (i.e. the light passing through is randomly and diffusely scattered by the particles) and a noticeable portion of the light that enters this translucent layer re-emerges via a radial scattered path. That is, there is backscatter, the redirection of the light waves back out to the open air.

Scattering takes place to a greater extent at shorter wavelengths. The longer wavelengths tend to pass straight through the translucent layer with unaltered paths of yellow light, and then encounter the next layer further back in the iris, which is a light absorber called the epithelium or uvea that is colored brownish-black. The brightness or intensity of scattered blue light that is scattered by the particles is due to this layer along with the turbid medium of particles within the stroma.

Thus, the longer wavelengths are not reflected (by scattering) back to the open air as much as the shorter wavelengths. Because the shorter wavelengths are the blue wavelengths, this gives rise to a blue hue in the light that comes out of the eye.1516 The blue iris is an example of a structural color because it relies only on the interference of light through the turbid medium to generate the color.

Blue eyes and brown eyes, therefore, are anatomically different from each other in a genetically non-variable way because of the difference between turbid media and melanin. Both kinds of eye color can remain functionally separate despite being "mixed" together.

Similar phenomena different from Tyndall scattering

When the day's sky is overcast, sunlight passes through the turbidity layer of the clouds, resulting in scattered, diffuse light on the ground (sunbeam). This exhibits Mie scattering instead of Tyndall scattering because the cloud droplets are larger than the wavelength of the light and scatters all colors approximately equally. When the daytime sky is cloudless, the sky's color is blue due to Rayleigh scattering instead of Tyndall scattering because the scattering particles are the air molecules, which are much smaller than the wavelengths of visible light.17 Similarly, the term Tyndall effect is incorrectly applied to light scattering by large, macroscopic dust particles in the air as due to their large size, they do not exhibit Tyndall scattering.18

Comparison between the three main scattering processes undergone by visible light
Scattering processParticle typeParticle sizeResulting effect
Rayleigh scatteringAir molecule (N2 and O2)< 1 nanometerSky blue hue
Tyndall scatteringColloidal particles in suspension50 nm to 1 μmBlue scattered light
Mie scatteringLarger air dust, or cloud droplets> 1 micrometerAll colors equally scattered

See also

References

  1. Helmenstine, Anne Marie (February 3, 2020). "Tyndall Effect Definition and Examples". ThoughtCo. https://www.thoughtco.com/definition-of-tyndall-effect-605756 ↩

  2. Helmenstine, Anne Marie (February 3, 2020). "Tyndall Effect Definition and Examples". ThoughtCo. https://www.thoughtco.com/definition-of-tyndall-effect-605756 ↩

  3. Helmenstine, Anne Marie (February 3, 2020). "Tyndall Effect Definition and Examples". ThoughtCo. https://www.thoughtco.com/definition-of-tyndall-effect-605756 ↩

  4. Helmenstine, Anne Marie (February 3, 2020). "Tyndall Effect Definition and Examples". ThoughtCo. https://www.thoughtco.com/definition-of-tyndall-effect-605756 ↩

  5. Reported in a 10-page biography of Tyndall by Arthur Whitmore Smith, a professor of physics, writing in an American scientific monthly in 1920; available online. /w/index.php?title=Arthur_Whitmore_Smith&action=edit&redlink=1 ↩

  6. "John Tyndall's blue sky apparatus". Royal Institution. Retrieved 2021-03-08. https://www.rigb.org/our-history/iconic-objects/iconic-objects-list/tyndall-blue-sky ↩

  7. "Richard Adolf Zsigmondy: Properties of Colloids". Nobel Lectures, Chemistry 1922–1941. Amsterdam: Elsevier Publishing Company. 1966. http://nobelprize.org/nobel_prizes/chemistry/laureates/1925/zsigmondy-lecture.html ↩

  8. Mappes, Timo; Jahr, Norbert; Csaki, Andrea; Vogler, Nadine; Popp, Jürgen; Fritzsche, Wolfgang (2012). "The Invention of Immersion Ultramicroscopy in 1912-The Birth of Nanotechnology?". Angewandte Chemie International Edition. 51 (45): 11208–11212. doi:10.1002/anie.201204688. PMID 23065955. /wiki/Doi_(identifier) ↩

  9. "Blue Sky and Rayleigh Scattering". HyperPhysics Concepts - Georgia State University. Retrieved 2021-03-08. http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c2 ↩

  10. "Blue Sky and Rayleigh Scattering". HyperPhysics Concepts - Georgia State University. Retrieved 2021-03-08. http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c2 ↩

  11. "Chemistry - Colloids". OpenStax. Archived from the original on Mar 7, 2021. Retrieved 2021-03-08. https://opentextbc.ca/chemistry/chapter/11-5-colloids/ ↩

  12. "Blue Sky and Rayleigh Scattering". HyperPhysics Concepts - Georgia State University. Retrieved 2021-03-08. http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c2 ↩

  13. Wriedt, Thomas (2002). "Using the T-Matrix Method for Light Scattering Computations by Non-axisymmetric Particles: Superellipsoids and Realistically Shaped Particles". Particle & Particle Systems Characterization. 19 (4): 256–268. doi:10.1002/1521-4117(200208)19:4<256::AID-PPSC256>3.0.CO;2-8. ISSN 1521-4117. https://onlinelibrary.wiley.com/doi/abs/10.1002/1521-4117%28200208%2919%3A4%3C256%3A%3AAID-PPSC256%3E3.0.CO%3B2-8 ↩

  14. Details on how blue eyes get their color [Mason, C. W., Blue Eyes, American Journal of Physical Chemistry, Vol. 28, Pages 500-501, 1924.] //archive.org/details/in.ernet.dli.2015.229329/page/n5/mode/2up ↩

  15. For a short overview of how the Tyndall Effect creates the blue and green colors in animals see uni-hannover.de http://www.itp.uni-hannover.de/~zawischa/ITP/scattering.html#tyndalleffekt ↩

  16. Sturm R.A. & Larsson M., Genetics of human iris color and patterns, Pigment Cell Melanoma Res, 22:544-562, 2009. ↩

  17. Smith, Glenn S. (2005). "Human color vision and the unsaturated blue color of the daytime sky". American Journal of Physics. 73 (7): 590–97. Bibcode:2005AmJPh..73..590S. doi:10.1119/1.1858479. /wiki/Bibcode_(identifier) ↩

  18. Helmenstine, Anne Marie (February 3, 2020). "Tyndall Effect Definition and Examples". ThoughtCo. https://www.thoughtco.com/definition-of-tyndall-effect-605756 ↩