Tyndall effect

The Tyndall effect is light scattering by particles in a colloid or in a very fine suspension. Also known as Willis–Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to the fourth power of the wavelength, so blue light is scattered much more strongly than red light. An example in everyday life is the blue colour sometimes seen in the smoke emitted by motorcycles, in particular two-stroke machines where the burnt engine oil provides these particles.

Flour suspended in water appears to be blue because only scattered light reaches the viewer and blue light is scattered by the flour particles more than red light.

Under the Tyndall effect, the longer wavelengths are more transmitted while the shorter wavelengths are more diffusely reflected via scattering. The Tyndall effect is seen when light-scattering particulate matter is dispersed in an otherwise light-transmitting medium, when the diameter of an individual particle is the range of roughly between 40 and 900 nm, i.e. somewhat below or near the wavelengths of visible light (400–750 nm).

It is particularly applicable to colloidal mixtures and fine suspensions; for example, the Tyndall effect is used in nephelometers to determine the size and density of particles in aerosols and other colloidal matter (see ultramicroscope and turbidimeter).

It is named after the 19th-century physicist John Tyndall.

Comparison with Rayleigh scattering
The Tyndall effect in opalescent glass: It appears blue from the side, but orange light shines through.[1]

Rayleigh scattering is defined by a mathematical formula that requires the light-scattering particles to be far smaller than the wavelength of the light. 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. 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, 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. Light scattering by particles of complex shape are described by the T-matrix method.

Blue irises
A blue iris

A blue iris in an eye is due to Tyndall scattering in a translucent layer in the iris. Brown and black irises have the same layer except with more melanin in it. The melanin absorbs light. In the absence of melanin, the layer is translucent (i.e. the light passing through is randomly and diffusely scattered) and a noticeable portion of the light that enters this translucent layer re-emerges via a scattered path. That is, there is backscatter, the redirection of the lightwaves back out to the open air. Scattering takes place to a greater extent at the shorter wavelengths. The longer wavelengths tend to pass straight through the translucent layer with unaltered paths, and then encounter the next layer further back in the iris, which is a light absorber. Thus, the longer wavelengths are not reflected (by scattering) back to the open air as much as the shorter wavelengths are. Because the shorter wavelengths are the blue wavelengths, this gives rise to a blue hue in the light that comes out of the eye.[2][3] The blue iris is an example of a structural color, in contradistinction to a pigment color.

Similar phenomena that are not Tyndall scattering

When the days sky is overcast, sunlight passes through the turbid layer of the clouds, resulting in scattered, diffuse light on the ground. 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.[4] On occasion, the term Tyndall effect is incorrectly applied to light scattering by large (macroscopic) dust particles in the air.

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See also

References
  1. "Blue & red | Causes of Color".
  2. For a short overview of how the Tyndall Effect creates the blue and green colors in animals see uni-hannover.de
  3. Sturm R.A. & Larsson M., Genetics of human iris colour and patterns, Pigment Cell Melanoma Res, 22:544-562, 2009.
  4. 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.
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