Photonic Crystal

A photonic crystal is a material structure whose internal units — such as nanocrystals — are arranged periodically at spacings comparable to the wavelength of light, so that its geometry alone determines which wavelengths are reflected and which pass through. The guanine crystals in chameleon skin work on this principle: when muscles change the spacing between crystals, the reflected color shifts as well. The same mechanism produces the iridescent 'structural color' seen in butterfly wings and opals.