Iridescence (also known asgoniochromism) is the phenomenon of certain surfaces that appeargradually to changecolour as the angle of view or the angle of illumination changes. Iridescence is caused bywave interference of light inmicrostructures orthin films. Examples of iridescence includesoap bubbles,feathers,butterfly wings and seashellnacre, and minerals such asopal.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.
The wordiridescence is derived in part from theGreek word ἶριςîris (gen. ἴριδοςíridos), meaningrainbow, and is combined with the Latin suffix-escent, meaning "having a tendency toward".[1] Iris in turn derives from the goddessIris ofGreek mythology, who is the personification of the rainbow and acted as a messenger of the gods.Goniochromism is derived from the Greek wordsgonia, meaning "angle", andchroma, meaning "colour".
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 asthin-film interference.
Iridescence is anoptical phenomenon of surfaces in whichhue 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 whichphase shift andinterference of the reflectionsmodulates the incidentallight, 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 tothin-film interference, the functional analogue of selective wavelength attenuation as seen with theFabry–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]
An iridescentbiofilm 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 bydiffraction. This is found in items like CDs, DVDs, some types ofprisms, orcloud iridescence.[7] 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 ofdiffraction gratings on the surface, such as the long rows of cells instriated muscle, or the specialized abdominal scales ofpeacock spiderMaratus robinsoni andM. chrysomelas.[8] 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 toplant pigments.[9][10][11]
In biological (andbiomimetic) uses, colours produced other than withpigments ordyes are calledstructural 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.[12] Structural colouration has been understood in general terms sinceRobert Hooke's 1665 bookMicrographia, where Hooke correctly noted that since the iridescence of apeacock'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 complexphotonic crystal.[15]
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 apearl-like luster.[16] Artificial pigments and paints showing an iridescent effect are often described as pearlescent, for example when used forcar paints.[17][18]
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'sBegonia 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 thelycophyteSelaginella and several species offerns.[27][28]
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.
^Graham, Rita M.; Lee, David W.; Norstog, Knut (1993). "Physical and Ultrastructural Basis of Blue Leaf Iridescence in Two Neotropical Ferns".American Journal of Botany.80 (2):198–203.doi:10.2307/2445040.JSTOR2445040.
^Picard, G.; Simon, D.; Kadiri, Y.; LeBreux, J. D.; Ghozayel, F. (3 October 2012). "Cellulose Nanocrystal Iridescence: A New Model".Langmuir.28 (41):14799–14807.doi:10.1021/la302982s.PMID22988816.
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