Iridescence

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Iridescence in soap bubbles

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 structural coloration (microstructures that interfere with light).

Etymology[edit]

Iridescent nightscape over La Silla Observatory.[1]

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."[2] 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[edit]

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.[3][4] 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).[3][5] 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,[6][7] or a wide range of colours can be observed.[8]

Iridescence can also be created by diffraction. This is found in items like CDs, DVDs, or cloud iridescence.[9] 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.[10][11][12]

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.[13][14] It was later found that iridescence in the peacock is due to a complex photonic crystal.[15]

Examples[edit]

Animals[edit]

Arthropods and molluscs[edit]

Chordates[edit]

The feathers of birds such as kingfishers,[16] Birds-of-paradise,[17] hummingbirds, parrots, starlings,[18] grackles, ducks, and peacocks[15] are iridescent. The lateral line on the Neon tetra is also iridescent.[6] A single iridescent species of gecko, Cnemaspis kolhapurensis, was identified in India in 2009.[19] The tapetum lucidum, present in the eyes of many vertebrates, is also iridescent.[20]

Meat[edit]

Minerals and compounds[edit]

Man made objects[edit]

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

See also[edit]

References[edit]

  1. ^ "Iridescent Nightscape over La Silla". Retrieved 12 June 2015. 
  2. ^ "Online Etymology Dictionary". etymonline.com. 
  3. ^ a b Nano-optics in the biological world: beetles, butterflies, birds and moths Srinivasarao, M. (1999) Chemical Reviews pp: 1935-1961
  4. ^ Physics of structural colours Kinoshita, S. et al (2008) Rep. Prog. Phys. 71: 076401
  5. ^ Iridescence: views from many angles Meadows, M. et al. (2009) J. R. Soc. Interface 6:S107-S113
  6. ^ a b Mechanism of variable structural colour in the neon tetra: quantitative evaluation of the Venetian blind model Yoshioka, S. et al. (2011) J. Roy. Soc. Interface 8: 56 - 66
  7. ^ Pterin pigments amplify iridescent ultraviolet signal in males of the orange sulphur butterfly, Colias eurytheme Rutowski, RL et al. (2005) Proc. R. Soc. B 272: 2329–2335
  8. ^ Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera) Saego, AE et al. (2009) J. R. Soc. Interface 6: S165-S184
  9. ^ 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
  10. ^ Nature's palette: the science of plant colour. Lee, DW (2007) University of Chicago Press
  11. ^ Iridescent flowers? Contribution of surface structures to optical signaling van der Kooi, CJ et al (2014) New Phytol 203: 667–673
  12. ^ Is floral iridescence a biologically relevant cue in plant–pollinator signaling? van der Kooi, CJ et al (2015) New Phytol 205: 18–20
  13. ^ Hooke, Robert. Micrographia. Chapter 36 ('Observ. XXXVI. Of Peacoks, Ducks, and Other Feathers of Changeable Colours.')
  14. ^ Ball, Philip (May 2012). "Scientific American". Nature's Color Tricks 306 (5): 74–79. doi:10.1038/scientificamerican0512-74. PMID 22550931. Retrieved April 23, 2012. 
  15. ^ a b Coloration strategies in peacock feathers Zi, J et al (2003) Proc. Natl. Acad. Sci. 100:12576–12578
  16. ^ Kingfisher feathers – colouration by pigments, spongy nanostructures and thin films Stavenga, D.G. et al. (2011) J. Exp. Biol. 214: 3960-3967
  17. ^ Dramatic colour changes in a bird of paradise caused by uniquely structured breast feather barbules Stavenga, D.G. et al. (2010) Proc. Roy. Soc. B. doi:10.1098/rspb.2010.2293
  18. ^ Plumage Reflectance and the Objective Assessment of Avian Sexual Dichromatism Cuthill, I.C. et al. (1999) Am. Nat. 153: 183-200
  19. ^ "New lizard species found in India". BBC Online. 24 July 2009. Retrieved 20 February 2014. 
  20. ^ Engelking, Larry (2002). Review of Veterinary Physiology. Teton NewMedia. p. 90. ISBN 1893441695. 

External links[edit]