Soap Bubble - foliage background - iridescent colours - Traquair 040801

Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change

color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...

as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers,

butterfly Butterflies are insects in the macrolepidopteran clade Rhopalocera from the order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The group comprise ...

wings and seashell nacre, and minerals such as

opal Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...

. It is a kind of

structural coloration 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 wi ...

that is due to

wave interference In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...

of light in

microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...

s or

thin films A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...

. 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 Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

word ἶρις ''îris'' ( gen. ἴριδος ''íridos''), meaning ''

rainbow A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows c ...

'', and is combined with the Latin suffix ''-escent'', meaning "having a tendency toward". Iris in turn derives from the goddess

Iris Iris most often refers to: *Iris (anatomy), part of the eye *Iris (mythology), a Greek goddess * ''Iris'' (plant), a genus of flowering plants * Iris (color), an ambiguous color term Iris or IRIS may also refer to: Arts and media Fictional ent ...

Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities ...

, who is the personification of the

and acted as a messenger of the gods. ''Goniochromism'' is derived from the Greek words ''gonia'', meaning "angle", and ''chroma'', meaning "colour".

Mechanisms

Iridescence is an

optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the sun or moon wit ...

of surfaces in which

hue In color theory, hue is one of the main properties (called color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that ...

changes with the angle of observation and the angle of illumination. It is often caused by multiple reflections from two or more semi-transparent surfaces in which

phase shift In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it ...

and

interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extr ...

of the reflections modulates the incidental

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

(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 due to

thin-film interference 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, either enhancing or reducing the reflected light. When the thickness of the film is an ...

, 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, Iridescent biofilm on a fishtank

Iridescence can also be created by diffraction. This is found in items like CDs, DVDs, some types of

prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...

s, or

cloud iridescence 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 photom ...

. 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 grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structur ...

s on the surface, such as the long rows of cells in

striated muscle Striations means a series of ridges, furrows or linear marks, and is used in several ways: * Glacial striation * Striation (fatigue), in material * Striation (geology), a ''striation'' as a result of a geological fault * Striation Valley, in An ...

, or the specialized abdominal scales of

peacock spider ''Maratus'' is a spider genus of the family Salticidae (jumping spiders). 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 wit ...

''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 coloration due to plant pigments. In biological (and

biomimetic Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...

) uses, colours produced other than with

pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...

s or dyes are called

. 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 ''Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. With Observations and Inquiries Thereupon.'' is a historically significant book by Robert Hooke about his observations through various lenses. It ...

'', where Hooke correctly noted that since the iridescence of a

peacock Peafowl is a common name for three bird species in the genera '' Pavo'' and '' Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female peafowl are r ...

's feather was lost when it was plunged into water, but reappeared when it was returned to the air, pigments could not be responsible.Hooke, Robert. Micrographia. Chapter 36 ('Observ. XXXVI. ''Of Peacoks, Ducks, and Other Feathers of Changeable Colours''.') It was later found that iridescence in the peacock is due to a complex

photonic crystal A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic ...

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 A pearl is a hard, glistening object produced within the soft tissue (specifically the mantle) of a living shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pearl is composed of calcium carb ...

-like luster. Artificial pigments and paints showing an iridescent effect are often described as pearlescent, for example when used for car paints.

Examples

Life

Invertebrates

File:Iridescent Insect Display.png, Cornell drawer displaying iridescent insects File:Female Golden Stag Beetle.jpg, The iridescent exoskeleton of a golden stag beetle File:Morpho didius Male Dos MHNT.jpg, Structurally coloured wings of ''

Morpho didius ''Morpho didius'', the giant blue morpho, is a Neotropical butterfly belonging to the subfamily Morphinae of family Nymphalidae. It is considered, by some authors, to be a subspecies of ''Morpho menelaus''. Description ''Morpho didius'' has ...

'' File:Haliotis iris LC0283.jpg, The inside surface of ''

Haliotis iris ''Haliotis iris'', common name paua, blackfoot paua or rainbow abalone, is a species of edible sea snail, a marine gastropod mollusk in the family Haliotidae, the abalones.Bouchet, P. (2014). Haliotis iris. Accessed through: World Register of Ma ...

'', the paua shell

Vertebrates

The feathers of birds such as

kingfisher Kingfishers are a family, the Alcedinidae, of small to medium-sized, brightly colored birds in the order Coraciiformes. They have a cosmopolitan distribution, with most species found in the tropical regions of Africa, Asia, and Oceania, ...

birds-of-paradise The birds-of-paradise are members of the family Paradisaeidae of the order Passeriformes. The majority of species are found in eastern Indonesia, Papua New Guinea and eastern Australia. The family has 44 species in 17 genera. The members of ...

, hummingbirds,

parrot Parrots, also known as psittacines (), are birds of the roughly 398 species in 92 genera comprising the order Psittaciformes (), found mostly in tropical and subtropical regions. The order is subdivided into three superfamilies: the Psittacoide ...

starling Starlings are small to medium-sized passerine birds in the family Sturnidae. The Sturnidae are named for the genus '' Sturnus'', which in turn comes from the Latin word for starling, ''sturnus''. Many Asian species, particularly the larger ones, ...

grackle Grackles is the common name of any of 11 passerine birds (10 extant and one extinct) native to North and South America. They belong to various genera in the icterid family. In all the species with this name, adult males have black or mostly blac ...

duck Duck is the common name for numerous species of waterfowl in the family Anatidae. Ducks are generally smaller and shorter-necked than swans and geese, which are members of the same family. Divided among several subfamilies, they are a form ...

s, and

s are iridescent. The lateral line on the

neon tetra The neon tetra (''Paracheirodon innesi'') is a Freshwater fish of the characin family (family Characidae) of order Characiformes. The type species of its genus, it is native to blackwater and clearwater streams in the Amazon basin of Sout ...

is also iridescent. A single iridescent species of gecko, '' Cnemaspis kolhapurensis'', was identified in India in 2009. The

tapetum lucidum The ''tapetum lucidum'' ( ; ; ) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light a ...

, present in the eyes of many vertebrates, is also iridescent. Iridescence is known to be present among non-avian dinosaurs such as

dromaeosaurids Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...

, enantiornithes, and

lithornithids Lithornithidae is an extinct, possibly paraphyletic (but see below) group of early paleognath birds. They are known from fossils dating to the Upper Paleocene through the Middle Eocene of North America and Europe, with possible Late Cretaceous r ...

. File:Peacock 2.jpg, Both the body and the train of the peacock are iridescent File:NeonTetra.JPG, A

File:Rainbow boa peruvian.jpg, The

rainbow boa The rainbow boa (''Epicrates cenchria'') is a boa species endemic to Central and South America. A semi-arboreal species (not only do they climb in they wild but also proven in captivity), it is known for its attractive iridescent/holographic ...

File:Nicobar Pigeon 820.jpg,

Nicobar pigeon The Nicobar pigeon (''Caloenas nicobarica'', Car: ') is a bird found on small islands and in coastal regions from the Andaman and Nicobar Islands, India, east through the Malay Archipelago, to the Solomons and Palau. It is the only living membe ...

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 The lycophytes, when broadly circumscribed, are a vascular plant (tracheophyte) subgroup of the kingdom Plantae. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldes ...

Selaginella ''Selaginella'' is the sole genus of vascular plants in the family Selaginellaceae, the spikemosses or lesser clubmosses. This family is distinguished from Lycopodiaceae (the clubmosses) by having scale-leaves bearing a ligule and by having ...

'' and several species of

ferns A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except th ...

. File:Iridescent begonia.jpg, Iridescent ''

Begonia ''Begonia'' is a genus of perennial flowering plants in the family Begoniaceae. The genus contains more than 2,000 different plant species. The Begonias are native to moist subtropical and tropical climates. Some species are commonly grown ind ...

'' leaf

Meat

File:Meat Iridiscence.png, Iridescence in meat, caused by light diffraction on the exposed muscle cells

Minerals and compounds

File:Wismut Kristall und 1cm3 Wuerfel.jpg, A

bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...

crystal with a thin iridescent layer of

bismuth oxide Bismuth(III) oxide is perhaps the most industrially important compound of bismuth. It is also a common starting point for bismuth chemistry. It is found naturally as the mineral bismite (monoclinic) and sphaerobismoite (tetragonal, much more rare ...

, with a whitish-silver bismuth cube for comparison File:Goethite-171990.jpg,

Goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

, an

iron(III) oxide-hydroxide Iron(III) oxide-hydroxide or ferric oxyhydroxideA. L. Mackay (1960): "β-Ferric Oxyhydroxide". ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 32, issue 250, pages 545-557. is the chemical compound of iron, oxygen, ...

, from Polk County, Arkansas File:Ladrador iridescence.jpg, Polished

labradorite Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect (schiller). Labradorite is an intermediate to calcic member of the plagioclase series. It ...

File:Engine oil rainbow p1120058.jpg, An

engine oil Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, detergen ...

spill File:Irid clouds1.jpg,

Cloud iridescence 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 photom ...

Man-made objects

File:Pearlescent Toyota Supra - 002.jpg, Pearlescent paint job on a Toyota Supra car File:CD-ROM.png, Playing surface of a

compact disc The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in Oc ...

File:Glitter nail polish %28purple%29.jpg, Iridescent

glitter Glitter is an assortment of small, reflective particles that come in a variety of shapes, sizes, and colors. Glitter particles reflect light at different angles, causing the surface to sparkle or shimmer. Glitter is similar to confetti, sparkle ...

nail polish File:Samsung Galaxy A50 back 2.jpg,

Smartphone A smartphone is a portable computer device that combines mobile telephone and computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities and extensive mobile operating systems, whic ...

with iridescent back panel

Nanocellulose Nanocellulose is a term referring to nano-structured cellulose. This may be either cellulose nanocrystal (CNC or NCC), cellulose nanofibers (CNF) also called nanofibrillated cellulose (NFC), or bacterial nanocellulose, which refers to nano-struc ...

is sometimes iridescent, as are thin films of

gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organi ...

and some other

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

s and alcohols when floating on water. To create jewelry with crystal glass that lets light refract in a rainbow spectrum, Swarovski coats some of its products with special metallic chemical coatings. For example, its ''Aurora Borealis'' gives the surface a rainbow appearance.

Optically variable ink Optically variable ink (OVI) also called color shifting ink is an anti- counterfeiting measure used on many major modern banknotes, as well as on other official documents ( professional licenses, for example). The ink displays two distinct colo ...

uses finely powdered iridescent glitter.

References

External links

A 2.2 MB GIF animation
of a morpho butterfly showing iridescence
"Article on butterfly iridescence"
{{Webarchive, url=https://web.archive.org/web/20151107012605/http://www.physics.org/featuredetail.asp?NewsId=22 , date=2015-11-07 Optics Optical phenomena Color