Editing Tyndall effect (section)

{{Short description|Scattering of light by tiny particles in a colloidal suspension}}
{{Distinguish|Rayleigh scattering}}

[[File:Why is the sky blue.jpg|thumb|alt=A piece of blue-looking opalescent glass, with orange light glowing in its shadow|The Tyndall effect in [[opalescent]] glass: it appears blue from the side, but orange light shines through.<ref name=":1">{{Cite web |last=Helmenstine |first=Anne Marie |date=February 3, 2020 |title=Tyndall Effect Definition and Examples |url=https://www.thoughtco.com/definition-of-tyndall-effect-605756 |access-date= |website=ThoughtCo |language=en}}</ref>]]

The '''Tyndall effect''' is [[light scattering by particles]] in a [[colloid]] such as a very fine [[suspension (chemistry)|suspension]] (a [[sol (chemistry)|sol]]). Also known as '''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|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.<ref name=":1" /> The same effect can also be observed with [[tobacco smoke]] whose fine particles also preferentially scatter blue light. 

Under the Tyndall effect, the longer wavelengths are [[Transmittance|transmitted]] more, while the shorter wavelengths are more [[diffuse reflection|diffusely reflected]] via [[scattering]].<ref name=":1" /> The Tyndall effect is seen when light-scattering [[particulates|particulate matter]] is dispersed in an otherwise light-transmitting medium, where the [[particle size|diameter]] of an individual [[particle]] is in the range of roughly 40 to 900 [[nanometre|nm]], i.e. somewhat below or near the wavelengths of [[visible spectrum|visible light]] (400–750 nm).

It is particularly applicable to colloidal mixtures; for example, the Tyndall effect is used in [[nephelometer]]s to determine the size and density of particles in [[aerosols]]<ref name=":1" /> and other colloidal matter. Investigation of the phenomenon led directly to the invention of the [[ultramicroscope]] and [[turbidimetry]].

It is named after the 19th-century physicist [[John Tyndall]], who first studied the phenomenon extensively.<ref name=":1" />