Iridescence

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Soap Bubble - foliage background - iridescent colours - Traquair 040801

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Dieselrainbow

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Iridescent biofilm on a fishtank

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Black pearl and his shell

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Iridescent Insect Display

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Female Golden Stag Beetle

Iridescence (also known as goniochromism) is a phenomenon in which the surfaces appear to gradually change color as the angle of view or the angle of illumination changes. The term derives from Iris, the Greek goddess of the rainbow, which is a fitting allusion to the broad spectrum of colors that iridescent surfaces display. Iridescence is not a property of the pigments themselves but is due to the structural arrangement of the surface, which leads to the selective reflection or scattering of light at different wavelengths.

Causes and Mechanisms

The phenomenon of iridescence is a result of the physical structure of the materials involved. It occurs when light waves are scattered by microstructures on a surface that are comparable in size to the wavelength of visible light. These structures interfere with the light, amplifying some wavelengths while canceling others out, depending on the viewing angle. This interference can be due to a variety of mechanisms, including diffraction gratings, thin-film interference, or the scattering of light by particles.

Thin-Film Interference

One of the most common causes of iridescence is thin-film interference, which occurs in materials that have thin, transparent layers. Light waves reflected from the top and bottom surfaces of these layers can interfere with each other, enhancing certain colors while reducing others. This effect is often seen in soap bubbles and oil slicks on water.

Diffraction Gratings

Diffraction gratings, which are surfaces that contain a series of closely spaced lines or grooves, can also produce iridescence. When light hits these structures, it is diffracted into several beams traveling in different directions. The angle at which light is diffracted depends on the spacing of the grooves and the wavelength of the light, leading to a spectrum of colors.

Structural Coloration

In nature, iridescence is a form of structural coloration, where the physical structure of an object, rather than pigments, determines the visible color. This is seen in the feathers of many birds, such as peacocks and hummingbirds, as well as in the scales of butterflies and beetles. These structures are often composed of microscopic arrays that reflect light in specific ways to produce vivid, changing colors.

Examples in Nature

Iridescence is widespread in the natural world and serves various functions, from camouflage and signaling to attracting mates. For instance, the iridescent plumage of a peacock is used in mating displays, while the shimmering of a butterfly's wings can help it evade predators by confusing their visual perception.

Applications

The principles of iridescence have been applied in various fields, including art, fashion, and technology. In art, iridescent materials can add a dynamic quality to works by changing appearance with the viewer's perspective. In fashion, fabrics with iridescent properties are prized for their ability to shift colors. Technologically, iridescence has inspired innovations such as anti-counterfeiting measures on currency and documents, where iridescent features are difficult to replicate.

See Also

• Structural coloration
• Diffraction grating
• Thin-film interference
• Color vision
• Optics