iridescence: one construct, many systems

In optics, iridescence is colour produced not by pigment but by structure: when light reflects from the top and bottom of a thin film, or from a regular stack of thin layers, the reflected waves interfere, reinforcing some wavelengths and cancelling others so the perceived colour shifts with viewing angle. Newton's Opticks described these colours in thin films and the ring patterns that still carry his name, and Kinoshita, Yoshioka, and Miyazaki's Physics of structural colors sets out the modern account across interference, diffraction, and scattering. The referent is angle-dependent colour arising from wave interference in sub-wavelength structure, the same mechanism that reappears on a beetle's wing-case and in a slab of labradorite.[1][2]

Unit

The wavelengths reinforced by interference, and how the colour shifts with viewing angle

Recovery means

Not applicable

Measured by

Characterised by reflectance spectra as a function of angle and by the layer spacing and refractive-index contrast that set the interference condition

In gemology the same effect gives certain stones their shifting rainbow sheen: labradorescence in labradorite, the flashes of fire agate and iris agate, and the surface play on some hematite all arise from interference at thin internal layers, twin lamellae, fractures, or films rather than from the body colour of the mineral. Nassau's The physics and chemistry of color catalogues these structural causes among the many ways minerals make colour, and Read's Gemmology describes how gemmologists recognise them at the bench. It is the identical interference construct as in optics, realised in a crystal's internal architecture.[3][4]

Unit

The sheen colour and the viewing angles over which it appears on a stone

Recovery means

Not applicable

Measured by

Judged by the colour, strength, and angular range of the sheen, and traced to the spacing of lamellae, films, or fractures inside the stone

Living things use the same physics to make colour: the metallic blues of Morpho butterflies, the shimmer of beetle elytra, peacock feathers, and some fish and plants arise from ordered nanostructures that interfere with light rather than from pigments. Vukusic and Sambles's Photonic structures in biology reviews how these architectures (multilayers, diffraction gratings, and photonic crystals) generate iridescent colour and how it serves signalling and camouflage. The construct is the same angle-dependent structural colour described in optics and seen in minerals, here evolved in tissue.[5]

Unit

The colours an organism's nanostructure reflects and how they change with angle

Recovery means

Not applicable

Measured by

Assessed by angle-resolved reflectance and by electron-microscope imaging of the periodic nanostructures responsible