The Tyndall effect, also known as Tyndall scattering, is the scattering of light by colloidal particles or particles in suspension. It is named after the 19th century 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 color sometimes seen in the smoke emitted by motorcycles, particularly two stroke machines where the burnt engine oil provides the particles.

The basis for distinguishing between Tyndall and Rayleigh scattering is that the former is defined as scattering by particulates in colloids, while the latter is defined as scattering by particles much smaller than the wavelength of the light, which may be individual atoms or molecules. Colloidal particulates are much bigger than atoms or molecules, and are in the rough vicinity of the size of a wavelength of light. It follows from scattering theory that Tyndall scattering (by colloidal particles) is much more intense than Rayleigh scattering (by atoms or molecules). The importance of the size factor for intensity can be seen in the large exponent it has in the mathematical statement of the intensity of Rayleigh scattering. There is no equivalent mathematical statement for Tyndall scattering. But if the colloid particles are spheroid, Tyndall scattering is mathematically analytical in terms of Mie theory, which admits particle sizes in the rough vicinity of the wavelength of light.

The term "Tyndall effect" is sometimes applied to light scattering by macroscopic particles such as dust in the air. However, this phenomenon is more like reflection, not scattering, as the macroscopic particles become clearly visible in the process.

Tyndall scattering is commercially exploited to determine the size and density of particles in colloids; see ultra-microscope and nephelometer.

When the iris in a person's eye is a blue color, the blue is due to Tyndall Scattering in a turbid layer in the iris. Brown and black irises have the same layer except they have more melanin in it. The melanin absorbs the light. In the absence of the melanin, the layer is translucent—i.e. the light passing through gets scattered—and a noticeable portion of the light that enters this turbid layer re-emerges via a scattered path. That is, there's back scatter, meaning redirection of the light waves 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 turbid layer with their paths largely unaltered, after which they encounter the next layer further back in the iris, and that layer absorbs light. Thus the longer wavelengths are not reflected (by scattering) back to the open air as much as the shorter wavelengths are. Since the shorter wavelengths are the blue wavelengths, this gives rise to a blue hue in the light that comes out of the eye. The blue iris is an example of a structural color, in contradistinction to a pigment color.

Raman scattering, an entirely different type of light scattering, may suffer interference from Tyndall scattering from the larger species in a mixture, in which case microfiltration to remove the larger species may improve Raman scattering measurements.

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