Iridescence

Iridescence in soap bubbles

"Pearlescent" redirects here. For the type of paint, see Pearlescent coatings.

"Iridescent" redirects here. For the Linkin Park song, see Iridescent (song).

Iridescence (also known as goniochromism) is the property of certain surfaces that appear to change colour as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, butterfly wings and sea shells, as well as certain minerals. It is often created by microstructures which interfere with light.

Description

Further information: Structural coloration, thin-film interference, and diffraction

Iridescence is an optical phenomenon of surfaces in which hue changes in proportion to the angles of observation and illumination. 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). This process, termed thin-film interference, is the functional analogue of selective wavelength attenuation as seen with the Fabry–Pérot interferometer. This is usually seen in plants and animals, soap bubbles, oil films on water, and many other items. In this case, the range of colours will often be rather narrow, usually shifting between two or three colours as the viewing angle changes, while changes in the thickness of the film will produce bands of colours that do not match the rainbow-spectrum, including browns, magentas, purples and blues.

However iridescence also occurs due to diffraction. This is found in items like CDs, DVDs, or cloud iridescence. In the case of diffraction, the entire rainbow of colours will typically be observed as the viewing angle changes. Iridescence from diffraction is rare in plants and animals, but does occur in some marine invertebrates, like seed shrimp or Burgess shale fossils. 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 in some types of flower petals.

In biological (and biomimetic) uses, colours are usually produced with pigments or dyes, so colours produced other than by pigment or dyes are called structural coloration. Microstructures, often multilayered, are often used to produce iridescence, as quite elaborate arrangements are needed to avoid reflecting different colours in different directions. Structural coloration 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.

Structural coloration can also be observed in meat cuts, smoked pork, packed poultry, bacon, roasted beef, and others may show a green or red coloration. This coloration is caused by the diffraction of light by the quasi-periodic structure of the muscular cell arrangement.. The coloration can be removed by roughening or drying the surface of the meat, or cutting at different angles where the diffraction does not produce visible colors.

Etymology

The word iridescence is derived in part from the Greek word ἶρις îris (gen. ἴριδος íridos), meaning rainbow, which 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".

Examples

Arthropods and molluscs

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  The iridescent exoskeleton of a golden stag beetle
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  Structurally coloured wings of Morpho didius
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  The inside surface of Haliotis iris, the paua shell
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  Structurally coloured wings of a Tachinid fly

Chordates

The feathers of birds such as kingfishers, hummingbirds, parrots, crows, ravens, starlings, grackles, ducks, and peacocks are iridescent. A single iridescent species of gecko, Cnemaspis kolhapurensis, was identified in India in 2009. The tapetum lucidum, present in the eyes of many vertebrates, is also iridescent.

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  Both the body and the train of the peacock are iridescent
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  A catfish
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  The rainbow boa

Minerals and compounds

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  An ammolite gemstone is a fossil ammonite.
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  A bismuth crystal with a thin iridescent layer of bismuth oxide
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  Goethite, an iron(III) oxide-hydroxide, from Polk County, Arkansas
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  An engine oil spill

Others

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  Cloud iridescence
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  Pearlescent paint jobs on a Toyota Supra car
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  Playing surface of a compact disc

Nanocellulose is sometimes iridescent, as are thin films of gasoline and some other hydrocarbons and alcohols when floating on water.

See also

• Anisotropy
• Bioluminescence, irrespective of angle
• Dichroic filter
• Dichroism
• Iridocyte
• Labradorescence (Adularescence)
• Opalescence
• Structural color
• Thin-film optics

External links

• Living photonic crystals
• A 2.2 MB GIF animation of a morpho butterfly showing iridescence
• "Article on butterfly iridescence"