Iridization

Iridization is an optical phenomenon that causes surfaces to subtly change color as the angle of view or illumination changes. This effect is commonly observed in various natural and artificial materials, producing a shimmering or rainbow-like appearance. The underlying principle behind iridization is thin-film interference. When light strikes a material with a very thin, transparent layer on its surface, the light waves reflect off both the top surface of the layer and the bottom surface of the layer. These two reflected waves interact. Depending on the thickness of the thin film and the wavelength of light, certain wavelengths will interfere constructively, appearing brighter, while others will interfere destructively, appearing dimmer. Because the path difference between the two reflected waves varies with the angle of observation, different colors become dominant at different angles, leading to the observed color shift.

Examples of iridization can be seen in the iridescent wings of some insects, the pearly sheen of mother-of-pearl, oil slicks on water, and certain types of synthetic materials like opals. The specific colors produced depend on the composition and thickness of the thin film responsible for the interference. In nature, this phenomenon can serve purposes such as camouflage, species recognition, or attracting mates. In manufactured goods, iridization is often intentionally created for aesthetic appeal.