irisation

Irisation, also called iridescence, is the optical phenomenon in which a surface displays a spectrum of colors that changes with the angle of light or the observer. Unlike colors produced by pigments, iridescent colors arise from structural interactions with light, such as interference or diffraction, rather than from chemical coloration.

The most common mechanisms are interference and diffraction. Thin-film interference occurs when light reflects off multiple closely spaced interfaces, producing constructive or destructive interference that depends on angle and film thickness. In layered or microstructured materials, light is scattered or refracted by periodic structures, as seen in nacre (mother-of-pearl), butterfly and beetle scales, and opals. Some forms of iridescence arise from diffraction by regular surface patterns or from photonic-crystal-like arrangements.

Natural examples include opals, nacreous shells, and iridescent butterfly wings, as well as certain beetle integuments. Artificial instances include soap bubbles, oil films on water, and reflective surfaces such as CDs or DVDs. Colors are often vivid but transient, changing as the viewing geometry or the film thickness evolves.

Irisation is studied for its scientific and practical interest, informing the design of biomimetic materials, optical coatings, and security features. The term is frequently used in francophone contexts, while iridescence is the more common English equivalent, though both describe the same class of angle-dependent coloration.