The morphology of moth and butterfly wing scales which exhibit reflective diffraction phenomena

The butterfly and moth families illustrate how nature has used diffractive micro-relief structures to achieve unique optical effects. Whereas the majority of insects use pigments (absorption) or occasionally thin film multi-layer structures (interference) to create colour, the wings of many families of butterfly and moth have complex 2-D or 3-D arrangements of submicron grating structures which produce zero and higher order diffraction conditions.The special properties of a diffraction grating can be understood by light incident perpendicularly on a transmission grating. The light is diffracted into a number of grating orders at angles θn given by the grating equation sin θn = n λ/d where λ is the wavelength of the light, n = 0, ± 1, --- and d is the grating period. The same conditions are valid for reflective diffraction structures. If the grating period is finer than the wavelength ie. d < λ, no first order diffraction exists for normal illumination.