Iridescence



Iridescence is an optical phenomenon characterized as the property of surfaces in which hue changes according to the angle from which the surface is viewed (as may be seen of soap bubbles and butterfly wings).

Iridescence is caused by multiple reflections from multi-layered, semi-transparent surfaces in which phase shift and interference of the reflections modulates the incident light (by amplifying or attenuating some frequencies more than others). This process is the functional analog of selective wavelength attenuation as seen with the Fabry-Pérot interferometer.

The word is derived in part from the Greek word iris (pl. irides), meaning "rainbow", which in turn derives from the goddess Iris of Greek mythology, who is the personification of the rainbow and acted as a messenger of the gods. Goniochromism is derived from the Greek words gonia which means angle, and chroma which means color. It is the property of certain surfaces to change their colour depending on the angle under which they are viewed. From a physics standpoint this is rather uncommon behaviour, and in man-made objects this is usually only found in paints that are specifically designed for the effect. Examples are pearlescent paints or interference pigments, which are sometimes used for custom paintjobs on cars. Some specially woven fabrics or certain shampoos can also exhibit such behaviour. Natural examples of goniochromism can be found in the appearance of certain insects, such as the Morpho butterfly, whose metallic-looking wings exhibit a colour shift depending on the observation angle.

Taxonomically, the most widespread distribution of iridescent tissue in animals may be the tapetum lucidum, present in the eyes of many vertebrate animals.

Because the apparent image changes with the angle of observation, iridescence is not fully reproduced by conventional still image photography; however, it can be reproduced by holography (which includes phase information).