iridescence

Iridescence is the optical phenomenon in which color changes with the angle of illumination or viewing. Unlike colors produced by pigments, iridescent colors arise from structural coloration: light is scattered, reflected, and interfered by micro- and nano-scale structures, so certain wavelengths are reinforced while others are canceled. The effect is most pronounced when multiple interfaces or layered materials create constructive interference at specific wavelengths, and shifts in angle alter the apparent color.

Natural occurrences include soap bubbles, which show rainbow colors as the film thins; bird and butterfly wing scales and beetle shells that contain periodic nanostructures; and the nacreous luster of some mollusk shells and opals. The coloration commonly depends on viewing geometry; these are angle-dependent, making iridescence different from pigments and from metallic surfaces that have more uniform sheen.

Mechanisms include thin-film interference in a single layer; multilayer interference in stacked lamellae; diffraction by periodic nanostructures; and photonic crystal effects in more complex arrangements. The refractive index contrast and the thickness of the structures determine the colors and their angular variation.

Biological roles vary: iridescence can play a role in communication and mate attraction, signaling, or camouflage. In humans, iridescent materials are used in security features, cosmetics, and coatings, while researchers study bio-inspired photonic structures to create durable, colorfast surfaces that do not rely on pigments.