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.