The highly effective optical means by which silvery fish, such as the European sardine and Atlantic herring, camouflage themselves from predators is explained this month in Nature Photonics. The investigation suggests that some species of fish have evolved a sophisticated skin structure that acts as a high quality reflector, regardless of the light’s wavelength, polarization or angle of incidence. This capability is not only essential for cloaking purposes but is very useful for controlling light in man-made photonic devices such as light-emitting diodes and optical waveguides.
The fish accomplish this much sought-after feat by having a skin composed of multiple layers of high-refractive-index guanine crystals separated by low-index layers of cytoplasm. Critically, Nicholas Roberts and co-workers have discovered that the skin actually contains two types of guanine crystal with different optical properties, allowing the reflection to be largely insensitive to polarization across a wide range of angles.
This polarization neutrality is thought to be especially important in the natural world, where several aquatic animals are known to have vision that can detect differences in polarization.
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