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".[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".
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]
Iridescence can also be created by
diffraction. This is found in items like CDs, DVDs, some types of
prisms, or
cloud 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 of
diffraction gratings on the surface, such as the long rows of cells in
striated muscle, or the specialized abdominal scales of
peacock spiderMaratus robinsoni and M. 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 to
plant pigments.[9][10][11]
In biological (and
biomimetic) uses, colours produced other than with
pigments or
dyes are called
structural colouration. 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.[12] 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.[13][14] It was later found that iridescence in the peacock is due to a complex
photonic crystal.[15]
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.[16] Artificial pigments and paints showing an iridescent effect are often described as pearlescent, for example when used for
car paints.[17]
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
lycophyteSelaginella and several species of
ferns.[25][26]
^Cuthill, I. C.; Bennett, A. T. D.; Partridge, J. C.; Maier, E. J. (February 1999). "Plumage Reflectance and the Objective Assessment of Avian Sexual Dichromatism". The American Naturalist. 153 (2): 183–200.
doi:
10.1086/303160.
JSTOR303160.
PMID29578758.
S2CID4386607.
^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.