Animal reflectors

All these reflectors work by interference of light in multilayer structures with dimensions less than a wavelength, so can be classed as photonic crystals.

The compound eyes of long-bodied decapod crustaceans, such as shrimps and lobsters, use mirrors in square boxes [1] Most nocturnal vertebrates have a reflecting tapetum lucidum behind the retina, which produces the 'eyeshine' seen in cats and dogs.

Land published a full analysis of an animal reflector that included electron microscopy, optical measurements, and a clear explanation of the theory [7] Animal multilayer reflectors work in the same way as a man-made dielectric mirror (or Bragg mirror) being composed of alternating layers of high and low refractive index, the thickness of each layer being 1/4 the wavelength most strongly reflected.

[9] Reflectors made of alternating layers of flat guanine crystals (refractive index, n = 1.83) and cytoplasm (n ≈ 1.33) have evolved independently in fish scales and in the tapeta of the eyes of elasmobranchs (Gur 2017).

contains remarkably regular arrays of rodlets, and reflects light from planes of the lattice by Bragg's Law (Fig.

A. The cells of the tapetum of the cat contain arrays of rodlets with a spacing at about λ/2. [ 2 ] B. Different lattice planes have different spacings and reflect different wavelengths. [ 3 ] C. The different domains of rodlets reflect different wavelengths to give an overall reflection with a wide spectral range.