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Iridescence

Article Discussion

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File:Soap Bubble - foliage background - iridescent colours - Traquair 040801.jpg
Iridescence in soap bubbles

Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstructures or thin films. Examples of iridescence include soap bubbles, feathers, butterfly wings and seashell nacre, and minerals such as opal. Pearlescence is a related effect where some or most of the reflected light is white. The term pearlescent is used to describe certain paint finishes, usually in the automotive industry, which actually produce iridescent effects.

EtymologyEdit

The word iridescence is derived in part from the Greek word ἶρις îris (gen. ἴριδος íridos), meaning rainbow, and is combined with the Latin suffix -escent, meaning "having a tendency toward".<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Iris in turn derives from the goddess Iris of Greek mythology, who is the personification of the rainbow and acted as a messenger of the gods. Goniochromism is derived from the Greek words gonia, meaning "angle", and chroma, meaning "colour".

MechanismsEdit

File:Dieselrainbow.jpg
Fuel on top of water creates a thin film, which interferes with the light, producing different colours. The different bands represent different thicknesses in the film. This phenomenon is known as thin-film interference.

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Iridescence is an optical phenomenon of surfaces in which hue changes with the angle of observation and the angle of illumination.<ref name="srinivasarao">Template:Cite journal</ref><ref>Template:Cite journal</ref> It is often caused by multiple reflections from two or more semi-transparent surfaces in which phase shift and interference of the reflections modulates the incidental light, by amplifying or attenuating some frequencies more than others.<ref name="srinivasarao" /><ref name="meadows">Template:Cite journal</ref> The thickness of the layers of the material determines the interference pattern. Iridescence can for example be due to thin-film interference, the functional analogue of selective wavelength attenuation as seen with the Fabry–Pérot interferometer, and can be seen in oil films on water and soap bubbles. Iridescence is also found in plants, animals and many other items. The range of colours of natural iridescent objects can be narrow, for example shifting between two or three colours as the viewing angle changes,<ref name="yoshioka">Template:Cite journal</ref><ref>Template:Cite journal</ref>

File:Iridescent biofilm on a fishtank.JPG
An iridescent biofilm on the surface of a fish tank diffracts the reflected light, displaying the entire spectrum of colours. Red is seen from longer angles of incidence than blue.

Iridescence can also be created by diffraction. This is found in items like CDs, DVDs, some types of prisms, or cloud iridescence.<ref>Template:Cite book</ref> In the case of diffraction, the entire rainbow of colours will typically be observed as the viewing angle changes. In biology, this type of iridescence results from the formation of diffraction gratings on the surface, such as the long rows of cells in striated muscle, or the specialized abdominal scales of peacock spider Maratus robinsoni and M. chrysomelas.<ref>Template:Cite journal</ref> Some types of flower petals can also generate a diffraction grating, but the iridescence is not visible to humans and flower-visiting insects as the diffraction signal is masked by the colouration due to plant pigments.<ref>Template:Cite bookTemplate:Pn</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

In biological (and biomimetic) uses, colours produced other than with pigments or dyes are called structural colouration. Microstructures, often multi-layered, are used to produce bright but sometimes non-iridescent colours: quite elaborate arrangements are needed to avoid reflecting different colours in different directions.<ref>Template:Cite journal</ref> Structural colouration has been understood in general terms since Robert Hooke's 1665 book Micrographia, where Hooke correctly noted that since the iridescence of a peacock's feather was lost when it was plunged into water, but reappeared when it was returned to the air, pigments could not be responsible.<ref name=HookeOnPeacockIridescence>Hooke, Robert. Micrographia. Chapter 36 ('Observ. XXXVI. Of Peacoks, Ducks, and Other Feathers of Changeable Colours.')</ref><ref name=Ball>Template:Cite journal</ref> It was later found that iridescence in the peacock is due to a complex photonic crystal.<ref name="peacock">Template:Cite journal</ref>

PearlescenceEdit

File:Black pearl and his shell.jpg
The pearlescent shell of a black-lip pearl oyster

Pearlescence is an effect related to iridescence and has a similar cause. Structures within a surface cause light to be reflected back, but in the case of pearlescence some or most of the light is white, giving the object a pearl-like luster.<ref name=pearleasence>Template:Cite book</ref> Artificial pigments and paints showing an iridescent effect are often described as pearlescent, for example when used for car paints.<ref name=paint>Template:Cite book</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

ExamplesEdit

LifeEdit

InvertebratesEdit

Eledone moschata has a bluish iridescence running along its body and tentacles.<ref>{{#invoke:citation/CS1|citation |CitationClass=web

}}</ref>

  • Iridescent Insect Display.png

    Cornell drawer displaying iridescent insects

  • Female Golden Stag Beetle.jpg

    The iridescent exoskeleton of a golden stag beetle

  • Morpho didius Male Dos MHNT.jpg

    Structurally coloured wings of Morpho didius

  • Eunice aphroditois.jpg

    The iridescent skin of a Bobbit worm, Eunice aphroditois

  • Haliotis iris LC0283.jpg

    The inside surface of Haliotis iris, the paua shell

VertebratesEdit

The feathers of birds such as kingfishers,<ref>Template:Cite journal</ref> birds-of-paradise,<ref>Template:Cite journal</ref> hummingbirds, parrots, starlings,<ref>Template:Cite journal</ref> grackles, ducks, and peacocks<ref name="peacock" /> are iridescent. The lateral line on the neon tetra is also iridescent.<ref name="yoshioka" /> A single iridescent species of gecko, Cnemaspis kolhapurensis, was identified in India in 2009.<ref>Template:Cite news</ref> The tapetum lucidum, present in the eyes of many vertebrates, is also iridescent.<ref>Template:Cite book</ref> Iridescence is known to be present among prehistoric non-avian and avian dinosaurs such as dromaeosaurids, enantiornithes, and lithornithids.<ref>Template:Cite journal</ref> Muscle tissues can display irisdescence. <ref>Template:Cite journal</ref>

  • Peacock 2.jpg

    Both the body and the train of the peacock are iridescent

  • NeonTetra.JPG

    A neon tetra

  • Rainbow boa peruvian.jpg

    The rainbow boa

  • Nicobar Pigeon 820.jpg

    Nicobar pigeon

PlantsEdit

Many groups of plants have developed iridescence as an adaptation to use more light in dark environments such as the lower levels of tropical forests. The leaves of Southeast Asia's Begonia pavonina, or peacock begonia, appear iridescent azure to human observers due to each leaf's thinly layered photosynthetic structures called iridoplasts that absorb and bend light much like a film of oil over water. Iridescences based on multiple layers of cells are also found in the lycophyte Selaginella and several species of ferns.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Non-biologicalEdit

MineralsEdit

MeteorologicalEdit

Human-madeEdit

  • Pearlescent Toyota Supra - 002.jpg

    Pearlescent paint job on a Toyota Supra car

  • 8 cd-da disc-to-1 mini mp3-cd.jpg

    Playing surface of compact discs

  • 1899 reverse.jpg

    Iridescent toning on the reverse of a Morgan dollar

  • Glitter nail polish (purple).jpg

    Iridescent glitter nail polish

  • Samsung Galaxy A50 back 2.jpg

    Smartphone with iridescent back panel

  • Engine oil rainbow p1120058.jpg

    An engine oil spill

  • Tempering standards used in blacksmithing.JPG

    Tempering colours are formed by heating steel, forming a thin oxide-film on the surface. The colour indicates the temperature it was heated to, making it one of the earliest practical uses of iridescence.

Nanocellulose is sometimes iridescent,<ref name="PicardSimon2012">Template:Cite journal</ref> as are thin films of petrol and some other hydrocarbons and alcohols when floating on water.<ref>Template:Cite book</ref>

See alsoEdit

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ReferencesEdit

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External linksEdit

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