Iridescence

Iridescence, or goniochromism, is an optical effect where a surface shifts in color depending on the angle from which it is viewed or lit. This happens because light waves interfere with each other when they reflect off microstructures or thin films. Common examples include soap bubbles, feathers, butterfly wings, the nacre of seashells, the mineral opal, and some automotive paint finishes. A related effect, pearlescence, occurs when most of the reflected light is white, giving a pearl-like luster; this is often what is meant by "pearlescent" paints.

The word "iridescence" comes from the Greek "iris" (rainbow) and the Latin suffix "-escent" (tending toward). Iris was the Greek goddess of the rainbow and a messenger of the gods. "Goniochromism" combines the Greek "gonia" (angle) and "chroma" (color).

The mechanism involves diffraction: the intensity of reflected light varies with direction and frequency. Multiple reflections from two or more semi-transparent surfaces cause phase shifts and interference, amplifying some light frequencies and weakening others. The thickness of the material's layers determines the resulting interference pattern. This is similar to thin-film interference seen in oil films on water and soap bubbles, or in a Fabry–Pérot interferometer. Natural iridescence can be narrow, shifting between just two or three colors as the viewing angle changes.

Iridescence can also come from diffraction by periodic structures, like the grooves on CDs and DVDs, some prisms, or cloud iridescence. In these cases, a full rainbow of colors often appears as the angle changes. In biology, this type arises from surface diffraction gratings, such as the rows of cells in striated muscle or the specialized scales of certain peacock spiders. Some flower petals have diffraction gratings too, but the iridescence is hidden from humans and insects by plant pigments.

Colors produced without pigments or dyes are called structural coloration. These often use multi-layered microstructures to create bright colors, sometimes non-iridescent, requiring careful arrangements to avoid shifting colors with angle. Robert Hooke noted in 1665 that a peacock feather lost its iridescence when wet and regained it when dry, proving pigments were not the cause. Later research showed the peacock's iridescence comes from a complex photonic crystal.

Pearlescence shares a similar cause: surface structures reflect light, but with much of it white, creating a pearl-like sheen. Artificial iridescent pigments and paints are often labeled pearlescent, especially in car finishes.

In nature, many plants use iridescence to adapt to dark environments. The leaves of *Begonia pavonina* appear iridescent blue due to layered photosynthetic structures called iridoplasts. Similar multi-layered iridescence appears in the lycophyte *Selaginella* and several ferns, like the blue oil fern. Some cave-dwelling bryophytes, such as the liverwort *Cyathodium cavernarum* and the mosses *Mittenia plumula* and *Schistostega pennata*, are also iridescent.

Among animals, the octopus *Eledone moschata* has a bluish iridescence on its body and tentacles. Bird feathers—from kingfishers, birds-of-paradise, hummingbirds, parrots, starlings, grackles, ducks, and peacocks—are iridescent due to ordered arrays of melanosomes in the barbules. Hollow melanosomes, mostly found in birds, boost color brightness by increasing the refractive index contrast between melanin, keratin, and air. The lateral line of the neon tetra is iridescent. A single iridescent gecko species, *Cnemaspis kolhapurensis*, was identified in India in 2009. The tapetum lucidum in many vertebrate eyes is also iridescent. Iridescence has been found in prehistoric non-avian and avian dinosaurs, including dromaeosaurids, enantiornithes, and lithornithids. Muscle tissue can also show iridescence.

Bacterial colonies of *Cellulophaga lytica* and *Flavobacterium* are iridescent. The cellular organization causing iridescence in *Flavobacterium* is thought to aid pack hunting.

Non-biological examples include minerals, meteorological phenomena, and human-made items like nanocellulose, as well as thin films of petrol and some other hydrocarbons and alcohols floating on water.