Riprendiamo il discorso dei metamateriali, cioè di quelle strutture che si comportano in modo non rispondente alla "logica". Per esempio l'opale ha a certe frequenze luminose un indice di rifrazione negativo: in pratica il raggio luminoso esce dal materiale invece di entrarvi ed esservi deviato (come se l'ombra del bastone nell'acqua flottasse nell'aria e vi venisse dispersa). Il risultato finale è un "giocare" con la luce che permette alla superficie di mostrare colori cangianti: tale effetto sfrutta la diffrazione della luce. La diffrazione is the phenomenon that occurs in the vicinity of a crack or a very small barrier (or rather, comparable to the wavelength of light), so that the light creates that particular rainbow effect, such as holograms in credit cards, or watching a CD bias, close to the tracks. We combine
now a second speech, which is that of layering, and self-assembly of materials, as in the shell of crab. Hence the need for protection is combined with the need for visibility, and the surface shows a color which is given purely by constructive interference of light waves with different structure of the material.
Nature has perfect structures, which come to be crystal photonic components, such as frustuli of diatoms, but also layered structures, where instead of the defective (or rather diversity) produces the effect. In the case of elytra of the beetles (which are the "semi-armor" that leave out the wings) of chitin layers with different refractive index (which in fact divert the light in a different way) cause quest'interferenza construction, then the quest'esaltazione color. And the mystery of nature, to think, comes from this wise alternation of order and disorder, or perfection and flaws.
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