Iridescence

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Iridescence in soap bubbles Soap Bubble - foliage background - iridescent colours - Traquair 040801.jpg
Iridescence in soap bubbles

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.

Contents

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. This phenomenon is known as thin-film interference. Dieselrainbow.jpg
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 as thin-film interference.

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]

An iridescent biofilm 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. Iridescent biofilm on a fishtank.JPG
An iridescent biofilm 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 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 spider Maratus 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

The pearlescent shell of a black-lip pearl oyster Black pearl and his shell.jpg
The pearlescent shell of a black-lip pearl oyster

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]

Examples

Life

Invertebrates

Eledone moschata has a bluish iridescence running along its body and tentacles. [18]

Vertebrates

The feathers of birds such as kingfishers, [19] birds-of-paradise, [20] hummingbirds, parrots, starlings, [21] grackles, ducks, and peacocks [15] 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. [22] The tapetum lucidum, present in the eyes of many vertebrates, is also iridescent. [23] Iridescence is known to be present among prehistoric non-avian and avian dinosaurs such as dromaeosaurids, enantiornithes, and lithornithids. [24]

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. 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 lycophyte Selaginella and several species of ferns. [25] [26]

Non-biological

Minerals

Meteorological

Human-made

Nanocellulose is sometimes iridescent, [27] as are thin films of petrol and some other hydrocarbons and alcohols when floating on water. [28]

See also

Related Research Articles

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In zoology, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopterans, scales are plates on the surface of the insect wing, and provide coloration. Scales are quite common and have evolved multiple times through convergent evolution, with varying structure and function.

<span class="mw-page-title-main">Diffraction grating</span> Optical component which splits light into several beams

In optics, a diffraction grating is an optical grating with a periodic structure that diffracts light into several beams traveling in different directions. The emerging coloration is a form of structural coloration. The directions or diffraction angles of these beams depend on the wave (light) incident angle to the diffraction grating, the spacing or distance between adjacent diffracting elements on the grating, and the wavelength of the incident light. The grating acts as a dispersive element. Because of this, diffraction gratings are commonly used in monochromators and spectrometers, but other applications are also possible such as optical encoders for high-precision motion control and wavefront measurement.

<span class="mw-page-title-main">Peafowl</span> Group of large game birds

Peafowl is a common name for two bird species in the genera Pavo and Afropavo within the tribe Pavonini of the family Phasianidae. Male peafowl are referred to as peacocks, and female peafowl are referred to as peahens.

<span class="mw-page-title-main">Chromatophore</span> Cells with a primary function of coloration found in a wide range of animals

Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, have a class of cells called melanocytes for coloration.

<span class="mw-page-title-main">Thin-film optics</span> Branch of optics that deals with very thin structured layers of different materials

Thin-film optics is the branch of optics that deals with very thin structured layers of different materials. In order to exhibit thin-film optics, the thickness of the layers of material must be similar to the coherence length; for visible light it is most often observed between 200 and 1000 nm of thickness. Layers at this scale can have remarkable reflective properties due to light wave interference and the difference in refractive index between the layers, the air, and the substrate. These effects alter the way the optic reflects and transmits light. This effect, known as thin-film interference, is observable in soap bubbles and oil slicks.

<i>Morpho</i> (genus) Genus of brush-footed butterflies

The morpho butterflies comprise many species of Neotropical butterfly under the genus Morpho. This genus includes more than 29 accepted species and 147 accepted subspecies, found mostly in South America, Mexico, and Central America. Morpho wingspans range from 7.5 cm (3.0 in) for M. rhodopteron to 20 cm (7.9 in) for M. hecuba, the imposing sunset morpho. The name morpho, meaning "changed" or "modified", is also an epithet. Blue morphos are severely threatened by the deforestation of tropical forests and habitat fragmentation. Humans provide a direct threat to this spectacular creature because their beauty attracts artists and collectors from all over the globe who wish to capture and display them. Aside from humans, birds like the jacamar and flycatcher are the adult butterfly’s natural predators.

<i>Morpho menelaus</i> Species of butterfly

The Menelaus blue morpho is one of thirty species of butterfly in the subfamily Morphinae. Its wingspan is approximately 12 cm (4.7"), and its dorsal forewings and hindwings are a bright, iridescent blue edged with black, while the ventral surfaces are brown. Its iridescent wings are an area of interest in research because of their unique microstructure. Due to its characteristic blue color, Morpho menelaus is considered valuable among collectors and was widely hunted in the 20th century.

<i>Gonepteryx rhamni</i> Species of butterfly

Gonepteryx rhamni, commonly named the common brimstone, is a butterfly of the family Pieridae. It lives throughout the Palearctic zone and is commonly found across Europe, Asia, and North Africa. Across much of its range, it is the only species of its genus, and is therefore simply known locally as the brimstone. Its wing span size is 60–74 mm (2.4–2.9 in). It should not be confused with the brimstone moth Opisthograptis luteolata.

<span class="mw-page-title-main">Melanosome</span> Organelle found in animal cells used for the synthesis, storage and transport of melanin

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<i>Hibiscus trionum</i> Species of flowering plant

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<span class="mw-page-title-main">Lawes's parotia</span> Species of bird

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<i>Maratus</i> Genus of spiders

Maratus is a spider genus of the family Salticidae. These spiders are commonly referred to as peacock spiders due to the males' colorful and usually iridescent patterns on the upper surface of the abdomen often enhanced with lateral flaps or bristles, which they display during courtship. Females lack these bright colors, being cryptic in appearance. In at least one species, Maratus vespertilio, the expansion of the flaps also occurs during ritualised contests between males. The male display and courtship dance are complex, involving visual and vibratory signals.

<i>Chrysochroa fulgidissima</i> Species of beetle

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<span class="mw-page-title-main">Animal coloration</span> General appearance of an animal

Animal colouration is the general appearance of an animal resulting from the reflection or emission of light from its surfaces. Some animals are brightly coloured, while others are hard to see. In some species, such as the peafowl, the male has strong patterns, conspicuous colours and is iridescent, while the female is far less visible.

<span class="mw-page-title-main">Cloud iridescence</span> Optical phenomenon

Cloud iridescence or irisation is a colorful optical phenomenon that occurs in a cloud and appears in the general proximity of the Sun or Moon. The colors resemble those seen in soap bubbles and oil on a water surface. It is a type of photometeor. This fairly common phenomenon is most often observed in altocumulus, cirrocumulus, lenticular, and cirrus clouds. They sometimes appear as bands parallel to the edge of the clouds. Iridescence is also seen in the much rarer polar stratospheric clouds, also called nacreous clouds.

<span class="mw-page-title-main">Thin-film interference</span> Optical phenomenon

Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, increasing reflection at some wavelengths and decreasing it at others. When white light is incident on a thin film, this effect produces colorful reflections.

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<span class="mw-page-title-main">Structural coloration</span> Colour in living creatures caused by interference effects

Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination with pigments. For example, peacock tail feathers are pigmented brown, but their microscopic structure makes them also reflect blue, turquoise, and green light, and they are often iridescent.

<span class="mw-page-title-main">Albinism</span> Disorder causing lack of pigmentation

Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and reddish pink or blue eyes. Individuals with the condition are referred to as albinos.

<span class="mw-page-title-main">Bio-inspired photonics</span>

Bio-inspired photonics or bio-inspired optical materials are the application of biomimicry to the field of photonics. This differs slightly from biophotonics which is the study and manipulation of light to observe its interactions with biology. One area that inspiration may be drawn from is structural color, which allows color to appear as a result of the detailed material structure. Other inspiration can be drawn from both static and dynamic camouflage in animals like the chameleon or some cephalopods. Scientists have also been looking to recreate the ability to absorb light using molecules from various plants and microorganisms. Pulling from these heavily evolved constructs allows engineers to improve and optimize existing photonic technologies, whilst also solving existing problems within this field.

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