Iridescence

Mechanisms

Iridescence is an optical phenomenon of surfaces in which hue changes with the angle of observation and the angle of illumination. This is fundamentally diffraction where the intensity of reflected light varies with direction and frequency. 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 described as thin-film, 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 from periodic objects such as CDs, DVDs, some types of prisms, or cloud iridescence. Often 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 specialised 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. 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

Plants

Many groups of plants have developed iridescence as an adaptation to dark environments. The leaves of Southeast Asia's Begonia pavonina, or peacock begonia, appear iridescent azure 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, such as the blue oil fern.

Several species of cave-dwelling bryophytes are iridescent, such as the liverwort Cyathodium cavernarum and the mosses Mittenia plumula and Schistostega pennata.

Animals

Invertebrates

Eledone moschata has a bluish iridescence running along its body and tentacles.

Vertebrates

The feathers of birds such as kingfishers, birds-of-paradise, hummingbirds, parrots, starlings, grackles, ducks, and peacocks are iridescent. The iridescence is created by ordered arrays of melanosomes within feather barbules; also hollow melanosomes, which are largely restricted to birds, enhance colour brightness by increasing the refractive-index contrast between melanin, keratin, and the air-filled central cavity of the organelle. 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.

Bacteria

Colonies of Cellulophaga lytica and Flavobacterium are iridescent. The same cellular organisation that creates iridescence in Flavobacterium is believed to help with pack hunting.

Non-biological

Minerals

Meteorological

Human-made

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