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The Tyndall effect is
light scattering by particles Light scattering by particles is the process by which small particles (e.g. ice crystals, dust, atmospheric particulates, cosmic dust, and blood cells) light scattering, scatter light causing optical phenomena such as the Rayleigh scattering, blue ...
in a
colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
or in a very fine
suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in mathematics * Suspension (chemistry), small solid particles suspend ...
. Also known as Tyndall scattering, it is similar to
Rayleigh scattering Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the ...
, in that the intensity of the scattered light is
inversely proportional In mathematics, two sequences of numbers, often experimental data, are proportional or directly proportional if their corresponding elements have a constant ratio, which is called the coefficient of proportionality or proportionality constan ...
to the fourth power of the
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
, so
blue light Blue Light or Blue light may refer to: Science and technology * Portion of the visible spectrum related to the blue color ** Blue laser ** Blue LED * Cherenkov radiation, the physical phenomenon responsible for the characteristic blue glow in nucl ...
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 A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle steered by a handlebar. Motorcycle design varies greatly to suit a range of different purposes: long-distance travel, commuting, cruising, ...
, in particular
two-stroke A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a Thermodynamic power cycle, power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being comple ...
machines where the burnt engine oil provides these particles. Under the Tyndall effect, the longer wavelengths are transmitted more while the shorter wavelengths are more diffusely reflected via
scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
. The Tyndall effect is seen when light-scattering
particulate matter Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...
is dispersed in an otherwise light-transmitting medium, where the
diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for ...
of an individual
particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object which can be described by several physical property, physical or chemical property, chemical ...
is in the range of roughly 40 to 900 nm, i.e. somewhat below or near the wavelengths of
visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
(400–750 nm). It is particularly applicable to colloidal mixtures and fine suspensions; for example, the Tyndall effect is used in
nephelometer A nephelometer or aerosol photometer is an instrument for measuring the concentration of suspended particulates in a liquid or gas colloid. A nephelometer measures suspended particulates by employing a light beam (source beam) and a light detec ...
s to determine the size and density of particles in
aerosols An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthrop ...
and other colloidal matter. Investigation of the phenomenon led directly to the
ultramicroscope An ultramicroscope is a microscope with a system that lights the object in a way that allows viewing of tiny particles via light scattering, and not light reflection or absorption. When the diameter of a particle is below or near the wavelength of ...
and
turbidimetry Turbidimetry (the name being derived from ''turbidity'') is the process of measuring the loss of intensity of transmitted light due to the scattering effect of particles suspended in it. Light is passed through a filter creating a light of known wav ...
. It is named after the 19th-century physicist
John Tyndall John Tyndall FRS (; 2 August 1820 – 4 December 1893) was a prominent 19th-century Irish physicist. His scientific fame arose in the 1850s from his study of diamagnetism. Later he made discoveries in the realms of infrared radiation and the p ...
, who first studied the phenomenon extensively.


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 Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...
had been removed, and the best way to detect these particulates was to bathe the air in intense light. 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. This observation enabled Tyndall to first propose the phenomenon which would later bear his name. In 1902 the
ultramicroscope An ultramicroscope is a microscope with a system that lights the object in a way that allows viewing of tiny particles via light scattering, and not light reflection or absorption. When the diameter of a particle is below or near the wavelength of ...
was developed by
Richard Adolf Zsigmondy Richard Adolf Zsigmondy ( hu, Zsigmondy Richárd Adolf; 1 April 1865 – 23 September 1929) was an Austrian-born chemist. He was known for his research in colloids, for which he was awarded the Nobel Prize in chemistry in 1925, as well as for co ...
(1865–1929) and
Henry Siedentopf Henry Friedrich Wilhelm Siedentopf (22 September 1872 in Bremen – 8 May 1940 in Jena) was a German physicist and pioneer of microscopy. Biography Siedentopf worked in Carl Zeiss company from 1899 to 1938. In 1907 he was nominated as the head ...
(1872–1940), working for
Carl Zeiss AG Carl Zeiss AG (), branded as ZEISS, is a German manufacturer of optical systems and optoelectronics, founded in Jena, Germany in 1846 by optician Carl Zeiss. Together with Ernst Abbe (joined 1866) and Otto Schott (joined 1884) he laid the f ...
. 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 Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
nanoparticles that generate the
cranberry glass Cranberry glass or Gold Ruby glass is a red glass made by adding gold salts or colloidal gold to molten glass. Tin, in the form of stannous chloride, is sometimes added in tiny amounts as a reducing agent. The glass is used primarily in expen ...
colour. This work led directly to Zsigmondy's
Nobel Prize for chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
.


Comparison with Rayleigh scattering

Rayleigh scattering Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the ...
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 A colloid is a mixture in which one substance consisting of microscopically dispersed Solubility, insoluble particles is suspension (chemistry), suspended throughout another substance. Some definitions specify that the particles must be disperse ...
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 A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has cir ...
, Tyndall scattering can be mathematically analyzed in terms of
Mie theory The Mie solution to Maxwell's equations (also known as the Lorenz–Mie solution, the Lorenz–Mie–Debye solution or Mie scattering) describes the scattering of an electromagnetic plane wave by a homogeneous sphere. The solution takes the f ...
, which admits particle sizes in the rough vicinity of the wavelength of light.
Light scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
by particles of complex shape are described by the
T-matrix method The Transition Matrix Method (T-matrix method, TMM) is a computational technique of light scattering by nonspherical particles originally formulated by Peter C. Waterman (1928–2012) in 1965. The technique is also known as null field method and ex ...
.


Blue irises

A blue
iris Iris most often refers to: *Iris (anatomy), part of the eye *Iris (mythology), a Greek goddess * ''Iris'' (plant), a genus of flowering plants *Iris (color), an ambiguous color term Iris or IRIS may also refer to: Arts and media Fictional enti ...
in an eye is blue due to Tyndall scattering by a translucent layer of turbid media in the iris that contains numerous small particles that are finely suspended within the fibrovasular structure of the stroma or front layer of the iris. Some brown irises have the same layer except with more amount of
melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
in it. Moderate amounts of melanin make hazel, dark blue & green eyes. However, most brown & black eyes are only colored by melanin because it is the only functionally important eye color which therefore lacks this turbid media due to its unnecessary function in protecting the eye from harmful UV rays. In eyes that contain both particles and melanin; melanin absorbs light. In the absence of melanin, the layer is
translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions a ...
(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 In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, a ...
, 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. The blue iris is an example of a
structural color Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination wit ...
because it relies only on the interference of light through the turbid medium to generate the color. Blue eyes & brown eyes therefore both seem to be anatomically different from each other because of the difference between turbid media and melanin. These differences are further evident through the fact that both kinds of eye color can remain functionally separate while also being mixed together.


Similar phenomena that are not Tyndall scattering

When the day's sky is
overcast Overcast or overcast weather, as defined by the World Meteorological Organization, is the meteorological condition of clouds obscuring at least 95% of the sky. However, the total cloud cover must not be entirely due to obscuring phenomena near t ...
,
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
passes through the
turbid Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids can ...
layer of the clouds, resulting in scattered, diffuse light on the ground (
sunbeam A sunbeam, in meteorological optics, is a beam of sunlight that appears to radiate from the position of the Sun. Shining through openings in clouds or between other objects such as mountains and buildings, these beams of particle-scattered sunl ...
). This exhibits
Mie scattering The Mie solution to Maxwell's equations (also known as the Lorenz–Mie solution, the Lorenz–Mie–Debye solution or Mie scattering) describes the scattering of an electromagnetic plane wave by a homogeneous sphere. The solution takes the f ...
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 Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the ...
instead of Tyndall scattering because the scattering particles are the air molecules, which are much smaller than the wavelengths of visible light. Similarly, the term ''Tyndall effect'' is incorrectly applied to light scattering by large,
macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic. Overview When applied to physical phenomena an ...
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...
particles in the air; however, due to their large size, they do not exhibit Tyndall scattering.


Gallery

File:Why is the sky blue.jpg, The Tyndall effect in
opalescent Opalescence refers to the optical phenomena displayed by the mineraloid gemstone opalopalescent. 2019. In Noah Webster's 1828 American Dictionary of the English Language. Retrieved January 7, 2019, from https://1828.mshaffer.com/d/word/opale ...
glass: It appears blue from the side, but orange light shines through. File:The Tyndall Effect in Fog.jpg, alt=, Fog scattering traffic light File:Koloidní povoha želatiny.jpg, alt=, The colloid on the right shows Tyndall effect while the solution does not File:Lyubertsy, A. A. Skochinsky Mining Institute - interior (16781901395).jpg, alt=, 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 Flour is a powder made by grinding raw grains, roots, beans, nuts, or seeds. Flours are used to make many different foods. Cereal flour, particularly wheat flour, is the main ingredient of bread, which is a staple food for many culture ...
suspended in
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
appears to be blue as only scattered light reaches the eyes and
blue light Blue Light or Blue light may refer to: Science and technology * Portion of the visible spectrum related to the blue color ** Blue laser ** Blue LED * Cherenkov radiation, the physical phenomenon responsible for the characteristic blue glow in nucl ...
is scattered by the flour particles more than red light


See also

*
Light scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
*
Transparency and translucency In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions ...
*
Ultramicroscope An ultramicroscope is a microscope with a system that lights the object in a way that allows viewing of tiny particles via light scattering, and not light reflection or absorption. When the diameter of a particle is below or near the wavelength of ...


References

{{Authority control Scattering Scattering, absorption and radiative transfer (optics) Smoke Optical phenomena