Editing Tyndall effect

{{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" />

== 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]].<ref>Reported in a 10-page biography of Tyndall by [[Arthur Whitmore Smith]], a professor of physics, writing in an American scientific monthly in 1920; [https://archive.org/stream/contributions05michuoft#page/n90/mode/1up available online].</ref> 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.<ref>{{Cite web |title=John Tyndall's blue sky apparatus |url=https://www.rigb.org/our-history/iconic-objects/iconic-objects-list/tyndall-blue-sky |access-date=2021-03-08 |website=Royal Institution |language=en}}</ref> 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]].<ref name=zmond>{{cite book | chapter=Richard Adolf Zsigmondy: Properties of Colloids | title=Nobel Lectures, Chemistry 1922–1941 | chapter-url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1925/zsigmondy-lecture.html | location=Amsterdam | publisher=Elsevier Publishing Company | year=1966 }}</ref><ref name=mappes12>{{cite journal |doi=10.1002/anie.201204688|pmid=23065955|title=The Invention of Immersion Ultramicroscopy in 1912-The Birth of Nanotechnology?|year=2012|last1=Mappes|first1=Timo|last2=Jahr|first2=Norbert|last3=Csaki|first3=Andrea|last4=Vogler|first4=Nadine|last5=Popp|first5=Jürgen|last6=Fritzsche|first6=Wolfgang|journal=Angewandte Chemie International Edition|volume=51|issue=45|pages=11208–11212}}</ref>

== 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.<ref name=":0">{{Cite web |title=Blue Sky and Rayleigh Scattering |url=http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c2 |access-date=2021-03-08 |website=HyperPhysics Concepts - Georgia State University}}</ref> For a dispersion of particles to qualify for the Rayleigh formula, the particle sizes need to be below roughly 40 nanometres (for visible light),{{Citation needed|date=March 2021}} and the particles may be individual molecules.<ref name=":0" /> [[Colloid]]al particles are bigger and are in the rough vicinity of the size of a wavelength of light. Tyndall scattering, i.e. colloidal particle scattering,<ref>{{Cite web |last= |title=Chemistry - Colloids |url=https://opentextbc.ca/chemistry/chapter/11-5-colloids/ |url-status=live |archive-url=https://web.archive.org/web/20210307070113/https://opentextbc.ca/chemistry/chapter/11-5-colloids/ |archive-date=Mar 7, 2021 |access-date=2021-03-08 |website= |publisher=OpenStax}}</ref> is much more intense than Rayleigh scattering due to the bigger particle sizes involved.{{Citation needed|date=March 2021}} 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.<ref name=":0" /> [[Light scattering]] by particles of complex shape are described by the [[T-matrix method]].<ref>{{Cite journal|last=Wriedt|first=Thomas|date=2002|title=Using the T-Matrix Method for Light Scattering Computations by Non-axisymmetric Particles: Superellipsoids and Realistically Shaped Particles|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/1521-4117%28200208%2919%3A4%3C256%3A%3AAID-PPSC256%3E3.0.CO%3B2-8|journal=Particle & Particle Systems Characterization|language=en|volume=19|issue=4|pages=256–268|doi=10.1002/1521-4117(200208)19:4<256::AID-PPSC256>3.0.CO;2-8|issn=1521-4117|url-access=subscription}}</ref>

== Blue irises ==
[[File:Iris close-up.jpg|A blue [[Iris (anatomy)|iris]] with some [[melanin]]|thumb]]

The color of blue [[eye]]s is due to the Tyndall [[scattering]] of light by a [[Transparency and translucency|translucent]] layer of [[Turbidity|turbid]] media in the [[Iris (anatomy)|iris]] containing numerous small particles of about 0.6 micrometers in diameter. These particles are finely suspended within the fibrovascular structure of the [[Stroma of iris|stroma]] or front layer of the iris.<ref>Details on how blue eyes get their color [Mason, C. W., Blue Eyes, [[iarchive:in.ernet.dli.2015.229329/page/n5/mode/2up|American Journal of Physical Chemistry]], Vol. 28, Pages 500-501, 1924.]</ref> 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.<ref>For a short overview of how the Tyndall Effect creates the blue and green colors in animals see [http://www.itp.uni-hannover.de/~zawischa/ITP/scattering.html#tyndalleffekt uni-hannover.de]</ref><ref>Sturm R.A. & Larsson M., Genetics of human iris color and patterns, Pigment Cell Melanoma Res, 22:544-562, 2009.</ref> 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.{{cn|date=December 2024}} Both kinds of eye color can remain functionally separate despite being "mixed" together.

== Similar phenomena different from Tyndall scattering ==
[[File:Tyndall Effect.jpg|thumb|[[Sunbeam]] exhibiting [[Mie scattering]] instead of Tyndall scattering.]]

When the day's sky is [[overcast]], [[sunlight]] passes through the [[turbid]]ity layer of the clouds, resulting in scattered, [[diffuse sky radiation|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.{{Citation needed|date=November 2022}} When the daytime sky is [[cloud cover|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.<ref>{{cite journal |title=Human color vision and the unsaturated blue color of the daytime sky |first=Glenn S. |last=Smith |journal=American Journal of Physics |volume=73 |issue=7 |pages=590–97 |year=2005|doi=10.1119/1.1858479 |bibcode=2005AmJPh..73..590S }}</ref> Similarly, the term ''Tyndall effect'' is incorrectly applied to light scattering by large, [[macroscopic scale|macroscopic]] [[dust]] particles in the air as due to their large size, they do not exhibit Tyndall scattering.<ref name=":1" />

{| class="wikitable"
|+ Comparison between the three main scattering processes undergone by visible light
|-
! Scattering process !! Particle type !! Particle size !! Resulting effect
|-
| Rayleigh scattering || Air molecule ({{chem2|N2}} and {{chem2|O2}}) || < 1 nanometer || Sky blue hue
|-
| Tyndall scattering || Colloidal particles in suspension || 50 nm to 1 μm || Blue scattered light
|-
| Mie scattering || Larger air dust, or cloud droplets || > 1 micrometer || All colors equally scattered
|}

== Gallery ==
<gallery mode="nolines">
File:The Tyndall Effect in Fog.jpg|alt=|Fog scattering traffic light<ref name=":1" />
File:Koloidní povoha želatiny.jpg|alt=|The colloid on the right shows Tyndall effect while the solution does not<ref name=":1" />
File:Lyubertsy, A. A. Skochinsky Mining Institute - interior (16781901395).jpg|alt=|Dust in the air exhibiting [[Mie scattering]] rather than Tyndall scattering
File:Lunar Laser McDonald Observatory.jpg|alt=|The path of the laser from the observatory becomes visible due to Tyndall effect
File:Internal Pantheon Light.JPG|alt=|Tyndall effect produced by the oculus in the top of the Pantheon's dome, Rome. The oculus is the only source of light inside the Pantheon.
File:WaterAndFlourSuspensionLiquid.jpg|[[Flour]] suspended in [[water]] appears to be blue as only [[light scattering by particles|scattered light]] reaches the eyes and [[Visible spectrum|blue light]] is scattered by the flour particles more than red light<ref name=":1" />
</gallery>

== See also ==
* [[Light scattering]]
* [[Transparency and translucency]]
* [[Ultramicroscope]]

== References ==
{{Reflist}}

{{Authority control}}
[[Category:Scattering]]
[[Category:Scattering, absorption and radiative transfer (optics)]]
[[Category:Smoke]]
[[Category:Optical phenomena]]