Iridescent

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.

Etymology

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". 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".

Mechanisms

Iridescence is an optical phenomenon of surfaces in which hue changes with the angle of observation and the angle of 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. 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,

Iridescence can also be created by diffraction. This is found in items like CDs, DVDs, some types of prisms, or cloud iridescence. 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''. 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.

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. 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.Hooke, Robert. Micrographia. Chapter 36 ('Observ. XXXVI. ''Of Peacoks, Ducks, and Other Feathers of Changeable Colours''.') It was later found that iridescence in the peacock is due to a complex photonic crystal.

Pearlescence

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. Artificial pigments and paints showing an iridescent effect are often described as pearlescent, for example when used for car paints.

Examples

Life

Invertebrates

'' Eledone moschata'' has a bluish iridescence running along its body and tentacles.
File:Iridescent Insect Display.png, Cornell drawer displaying iridescent insects
File:Female Golden Stag Beetle.jpg, The iridescent exoskeleton of a golden stag beetle
File:Morpho didius Male Dos MHNT.jpg, Structurally coloured wings of '' Morpho didius''
File:Eunice aphroditois.jpg, The iridescent skin of a Bobbit worm, ''Eunice aphroditois''
File:Haliotis iris LC0283.jpg, The inside surface of '' Haliotis iris'', the paua shell

Vertebrates

The feathers of birds such as kingfishers, birds-of-paradise, hummingbirds, parrots, starlings, grackles, ducks, and peacocks are iridescent. The lateral line on the neon tetra is also 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. Iridescence is known to be present among prehistoric non-avian and avian dinosaurs such as dromaeosaurids, enantiornithes, and lithornithids. Muscle tissues can display irisdescence.

File:Peacock 2.jpg, Both the body and the train of the peacock are iridescent
File:NeonTetra.JPG, A neon tetra
File:Rainbow boa peruvian.jpg, The rainbow boa
File:Nicobar Pigeon 820.jpg, Nicobar pigeon

Plants

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.

File:Iridescent begonia.jpg, Iridescent ''Begonia'' leaf
File:Peacock Fern (Selaginella wildenowii) (8681119528).jpg, '' Selaginella wildenowii'' leaves
File:Pollia.jpg, '' Pollia condensata'' fruits
File:Ophrys speculum-IMG 0321.jpg, '' Ophrys speculum'' flowers

Non-biological

Minerals

File:Wismut Kristall und 1cm3 Wuerfel.jpg, A bismuth crystal with a thin iridescent layer of bismuth oxide, with a whitish-silver bismuth cube for comparison
File:Goethite-171990.jpg, Goethite, an iron(III) oxide-hydroxide, from Polk County, Arkansas
File:Ladrador iridescence.jpg, Polished labradorite

Meteorological

File:Polar Stratospheric Clouds.jpg, Polar stratospheric clouds displaying a Nacreous iridescence
File:Irid clouds1.jpg, Cloud iridescence

Human-made

File:Pearlescent Toyota Supra - 002.jpg, Pearlescent paint job on a Toyota Supra car
File:8 cd-da disc-to-1 mini mp3-cd.jpg, Playing surface of compact discs
File:1899 reverse.jpg, Iridescent toning on the reverse of a Morgan dollar
File:Glitter nail polish (purple).jpg, Iridescent glitter nail polish
File:Samsung Galaxy A50 back 2.jpg, Smartphone with iridescent back panel
File:Engine oil rainbow p1120058.jpg, An engine oil spill
File: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, as are thin films of petrol and some other hydrocarbons and alcohols when floating on water.

See also

* Anisotropy
* Bioluminescence, irrespective of angle
* Dichroic filter
* Dichroism
* Iridocyte
* Labradorescence (Adularescence)
* Metallic colour
* Opalescence
* Structural colour
* Thin-film optics
* Nacre
* Opal

References

External links

A 2.2 MB GIF animation
of a morpho butterfly showing iridescence

"Article on butterfly iridescence"
{{Webarchive, url=https://web.archive.org/web/20151107012605/http://www.physics.org/featuredetail.asp?NewsId=22 , date=2015-11-07

Optics
Optical phenomena
Color