The
Tyndall effect, also known as
Tyndall scattering, is
light scatteringLight scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
by particles in a
colloidA colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
or particles in a fine
suspensionIn chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
. It is named after the 19th century physicist
John TyndallJohn Tyndall FRS was a prominent Irish 19th century physicist. His initial scientific fame arose in the 1850s from his study of diamagnetism. Later he studied thermal radiation, and produced a number of discoveries about processes in the atmosphere...
. It is similar to
Rayleigh scatteringRayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
, in that the intensity of the scattered light depends on the fourth power of the frequency, so 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, particularly two stroke machines where the burnt engine oil provides the particles.
Under the Tyndall effect, the longer-wavelength light is more transmitted while the shorter-wavelength light is more reflected via scattering. An analogy to this wavelength dependency is that longwave electromagnetic waves such as radio waves are able to pass through the walls of buildings, while shortwave electromagnetic waves such as light waves are stopped and reflected by the walls. The Tyndall effect is seen when light-scattering particulate-matter is dispersed in an otherwise light-transmitting medium, when the cross-section of an individual particulate is the range of roughly between 40 and 900 nanometers, i.e., somewhat below or near the wavelength of visible light (400–750 nanometers).
The Tyndall effect is commercially exploited to determine the size and density of particles in aerosols and other colloidal matter; see
ultramicroscopeAn ultramicroscope is a system of illumination for viewing tiny particles. When the diameter of a particle is below or near the wavelength of light , the particle cannot be seen in a light microscope with the usual method of illumination. The ultramicroscope system is based on light scattering, not...
and turbidimeter.
Difference from 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. For a dispersion of particles to qualify for the Rayleigh formula, the particle sizes need to be below roughly 40 nanometers; and the particles may be individual molecules. Colloidal particles are bigger, 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. 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 scatteringRayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
. There is no equivalent mathematical statement for Tyndall scattering. But, if the colloid particles are
spheroidA spheroid, or ellipsoid of revolution 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....
, Tyndall scattering is mathematically analysable in terms of
Mie theoryThe Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...
, which admits particle sizes in the rough vicinity of the wavelength of light.
Blue irises
A blue
irisThe iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...
in an eye is due to Tyndall scattering in a
turbidTurbidity is the cloudiness or haziness of a fluid caused by 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....
layer in the iris. Brown and black irises have the same layer except with more
melaninMelanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
in it. The melanin absorbs light. In the absence of melanin, the layer is translucent (i.e., the light passing through is randomly and diffusely scattered) and a noticeable portion of the light that enters this turbid layer re-emerges via a scattered path. That is, there is
backscatterIn physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...
, the redirection of the lightwaves 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 unaltered paths, and then encounter the next layer further back in the iris, which is a light absorber. 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 colorA pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
.
Weather
On a day when the sky is overcast, the sunlight passes through the turbid layer of the clouds, resulting in scattered,
diffuse light on the groundDiffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky...
. This does not exhibit Tyndall scattering because the cloud droplets are larger than the wavelength of light and scatter all colors approximately equally. On a day when the sky is cloud-free, the sky's color is blue in consequence of light scattering, but this is not termed Tyndall scattering because the scattering particles are the molecules of the air, which are much smaller than the wavelength of the light. On occasion, the term
Tyndall effect is incorrectly applied to light scattering by macroscopic dust particles in the air. However, this is more like
reflectionReflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
, not
scatteringScattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
, as the macroscopic particles become clearly visible in the process.
See also
- Light scattering
Light scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
- Nephelometer; aka turbidimeter
A nephelometer is a stationary or portable instrument for measuring suspended particulates in a liquid or gas colloid. A nephelometer measures suspended particulates by employing a light beam and a light detector set to one side of the source beam. Particle density is then a function of the...
- Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
- Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...
- Ultramicroscope
An ultramicroscope is a system of illumination for viewing tiny particles. When the diameter of a particle is below or near the wavelength of light , the particle cannot be seen in a light microscope with the usual method of illumination. The ultramicroscope system is based on light scattering, not...