- Birds, butterflies, squid and other creatures often sport intense or changing colors that are not formed by pigments but by highly organized nanostructures that researchers are only beginning to unravel.
- Orderly and disorderly geometric patterns of these nanostructures reflect only certain wavelengths of light, creating specific colors that in some cases can also shift if the structures get wet or if their dimensions change.
- Scientists are making synthetic materials that mimic these biological structures, which could lead to cars or dresses that change color as they move, sensors that detect impurities in drinking water, efficient optical chips for cell phones and authentication marks on credit cards that are exceedingly hard to counterfeit.
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The changing hues of a peacock’s splendid tail feathers have always captivated curious minds. Seventeenth-century English scientist Robert Hooke called them “fantastical,” in part because wetting the feathers caused the colors to disappear. Hooke used the recently invented microscope to investigate the feathers and saw that they were covered with tiny ridges, which he figured might produce the brilliant yellows, greens and blues.
Hooke was on the right track. The intense colors of bird plumages, butterfly wings and the bodies of squid are often produced not by light-absorbing pigments but by arrays of tiny structures that are just a few hundred nanometers wide. The size and spacing of these structures pick out particular wavelengths from the full spectrum of sunlight. The hues are also often iridescent, changing, like magic, from blue to green or orange to yellow, depending on the angle at which we see the animal. And because the colors are produced just by reflecting light rather than absorbing some of it, as pigments do, they can be more brilliant. The blue morpho butterfly of South and Central America can be seen from up to a kilometer away; it seems to shine when sunlight penetrates a tropical forest canopy and bounces off its wings.