lo.tangelini / Creative CommonsWhat differentiates the colors is the wavelength of the light, which in the visible spectrum of the human eye varies between 400 nanometers (blue/violet) and 700 nanometers (dark red). One nanometer corresponds to one millionth of a millimeter. Objects like drops of steam or soap bubbles that measure a fraction of 1 millimeter spread the light that falls on them. That is why clouds in the sky and the foam on beer are white. Structures that are much smaller than the wavelength of visible light, measuring 1 nanometer or less, such as simple molecules, spread very little light. They can also absorb light: beer is yellow although its foam is white because the molecules that the beer consists of take away the other colors and allow the center of the spectrum, the yellow, to pass through. On the scale comparable to wavelength, the particles cause interference with the light if they are arranged in an organized, regular pattern, like the slats of window blinds, for example. This effect gives rise to the multicolored appearance of the recorded side of a CD, whose grooves are this size, and the iridescence of butterfly wings or peacock feathers that are covered by nanometer-sized fibers separated by spaces of similar dimension. This arrangement is essential so that nanostructures are able to cause interference with light.
Luiz Nunes de Oliveira, São Carlos Institute of Physics, USP
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