Tyndall effect

The Tyndall effect is an effect of light scattering by colloidal particles or particles in suspension. It is named after the 19th century Irish scientist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth power of the frequency, so blue light is scattered more strongly than red light. An example in everyday life is the blue colour sometimes seen in the smoke emitted by motor bikes. The phenomenon is best explained by Mie theory as the particle size is much greater than the wavelength of light.

The basis for distinguishing between Tyndall and Rayleigh scattering is the intensity of the phenomenon. Colloidal particles are much larger than atoms or molecules. It follows from scattering theory that Tyndall scattering (by colloidal particles) is much more intense than Rayleigh scattering (by atoms or molecules). Tyndall scattering can be used to determine the size of colloidal particles. Tyndall scattering is also often used to describe light scattering by macroscopic particles such as dust in the air. However, this phenomenon is more like reflection, as the macroscopic particles become visible in the process.

When measuring Raman spectra of solutions, ultrafiltration may be needed to minimize interference by Tyndall scattering.

Prolonged and brilliantly coloured sunsets and twilights were frequently seen in London in the summer of 1815 due to Tyndall scattering of sunlight by ash particles in the upper atmosphere, produced by the earlier eruption of the volcano Tambora. Such a sunset may have been the inspiration for the brilliant and evocative sunset in the painting The Fighting Temeraire, though a similar effect could have been produced by Rayleigh scattering with exceptionally high atmospheric pressure.

See also

* [http://silver-lightning.com/tyndall/ demonstration]
* John Tyndall
* Rayleigh scattering
* Diffuse sky radiation ("Why is the sky blue?")
* Brownian motion, physical phenomenon where minute particles immersed in a fluid move about randomly.
* Crepuscular rays
* Volumetric lighting