Goniochromism

Goniochromism is a phenomenon where the color of a material changes depending on the angle at which it is viewed. This is a type of structural coloration, meaning the color arises from the physical structure of the material rather than from pigments or dyes. The effect is caused by the interference of light waves reflecting off different layers or surfaces within the material. As the viewing angle changes, the path length of the light rays also changes, leading to constructive or destructive interference at different wavelengths, thus altering the perceived color.

Common examples of goniochromism can be observed in nature, such as the iridescent colors of butterfly wings, peacock feathers, and the shells of some mollusks. In manufactured products, goniochromic effects are often found in special effect paints used for automobiles and on security features of currency and credit cards. These features are designed to be difficult to counterfeit due to their unique angular color-shifting properties. The precise color change and the range of angles over which it occurs are determined by the precise dimensions and arrangement of the nanostructures within the material.