Iridescence

Iridescence in soap bubbles

Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually 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 structural coloration (microstructures that interfere with light).

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".[1] 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

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

Iridescence is an optical phenomenon of surfaces in which hue changes with the angle of observation and the angle of illumination.[2][3] 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).[2][4] 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,[5][6] or a wide range of colours can be observed.[7]

Iridescence can also be created by diffraction. This is found in items like CDs, DVDs, some types of prisms, or cloud iridescence.[8] 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. 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 coloration due to plant pigments.[9][10][11]

In biological (and biomimetic) uses, colours produced other than with pigments or dyes are called structural coloration. Microstructures, often multilayered, are used to produce bright but sometimes non-iridescent colours: 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.[12][13] It was later found that iridescence in the peacock is due to a complex photonic crystal.[14]

Examples

Life

Arthropods and molluscs

Chordates

The feathers of birds such as kingfishers,[15] Birds-of-paradise,[16] hummingbirds, parrots, starlings,[17] grackles, ducks, and peacocks[14] are iridescent. The lateral line on the Neon tetra is also iridescent.[5] A single iridescent species of gecko, Cnemaspis kolhapurensis, was identified in India in 2009.[18] The tapetum lucidum, present in the eyes of many vertebrates, is also iridescent.[19]

Meat

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.[20]

Minerals and compounds

Man-made objects

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

To create jewelry with crystal glass that lets light refract in a rainbow spectrum, Swarovski coats some of its products with special metallic chemical coatings and for example, Aurora Borealis, or "AB", gives the surface a rainbow appearance. Rainbow anodizing is also possible.

Iridescent products in the cosmetic and clothing industries are often referred as being holographic.

See also

References

  1. ↑ "Online Etymology Dictionary". etymonline.com.
  2. 1 2 Nano-optics in the biological world: beetles, butterflies, birds and moths Srinivasarao, M. (1999) Chemical Reviews pp: 1935–1961
  3. ↑ Physics of structural colours Kinoshita, S. et al (2008) Rep. Prog. Phys. 71: 076401
  4. ↑ Meadows M.; et al. (2009). "Iridescence: views from many angles". J. R. Soc. Interface. 6: S107–S113. doi:10.1098/rsif.2009.0013.focus.
  5. 1 2 Yoshioka S.; et al. (2011). "Mechanism of variable structural colour in the neon tetra: quantitative evaluation of the Venetian blind model". J. Roy. Soc. Interface. 8 (54): 56–66. doi:10.1098/rsif.2010.0253.
  6. ↑ Rutowski RL; et al. (2005). "Pterin pigments amplify iridescent ultraviolet signal in males of the orange sulphur butterfly, Colias eurytheme" (PDF). Proc. R. Soc. B. 272 (1578): 2329–2335. PMC 1560183 Freely accessible. PMID 16191648. doi:10.1098/rspb.2005.3216.
  7. ↑ Saego AE; et al. (2009). "Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)". J. R. Soc. Interface. 6: S165–S184.
  8. ↑ Meteorology By Professor of Atmospheric and Oceanic Sciences University of Wisconsin-Madison Director Cooperative Institute for Meteorological Satellite Studies (Cimss) Steven A Ackerman, Steven A. Ackerman, John A. Knox -- Jones and Bartlett Learning 2013 Page 173-175
  9. ↑ Nature's palette: the science of plant colour. Lee, DW (2007) University of Chicago Press
  10. ↑ Iridescent flowers? Contribution of surface structures to optical signaling van der Kooi, CJ et al (2014) New Phytol 203: 667–673
  11. ↑ Is floral iridescence a biologically relevant cue in plant–pollinator signaling? van der Kooi, CJ et al (2015) New Phytol 205: 18–20
  12. ↑ Hooke, Robert. Micrographia. Chapter 36 ('Observ. XXXVI. Of Peacoks, Ducks, and Other Feathers of Changeable Colours.')
  13. ↑ Ball, Philip (May 2012). "Nature's Color Tricks". Scientific American. 306 (5): 74–79. PMID 22550931. doi:10.1038/scientificamerican0512-74. Retrieved April 23, 2012.
  14. 1 2 Zi J; et al. (2003). "Coloration strategies in peacock feathers". Proc. Natl. Acad. Sci. 100 (22): 12576–12578. PMC 240659 Freely accessible. PMID 14557541. doi:10.1073/pnas.2133313100.
  15. ↑ Stavenga D.G.; et al. (2011). "Kingfisher feathers – colouration by pigments, spongy nanostructures and thin films". J. Exp. Biol. 214 (23): 3960–3967. doi:10.1242/jeb.062620.
  16. ↑ Stavenga D.G.; et al. (2010). "Dramatic colour changes in a bird of paradise caused by uniquely structured breast feather barbules" (PDF). Proc. Roy. Soc. B. 278 (1715): 2098–2104. PMC 3107630 Freely accessible. PMID 21159676. doi:10.1098/rspb.2010.2293.
  17. ↑ Plumage Reflectance and the Objective Assessment of Avian Sexual Dichromatism Cuthill, I.C. et al. (1999) Am. Nat. 153: 183-200 JSTOR 303160
  18. ↑ "New lizard species found in India". BBC Online. 24 July 2009. Retrieved 20 February 2014.
  19. ↑ Engelking, Larry (2002). Review of Veterinary Physiology. Teton NewMedia. p. 90. ISBN 1-893441-69-5.
  20. ↑ Sophie (13 March 2014). "When plants glow". naturanaute.com.
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