Color Without Pigment

Color Without Pigment

Pigments absorb light in most wavelengths and reflect back in just a few, which our eyes detect as "color." But that's not the only way for things to appear colored to us. Rainbows result from clear raindrops, for instance, and from the mysterious habit of light to bend when it passes through the barrier between one clear medium and another, together with the even more curious fact that light of longer wavelengths (like red) bends more than that of shorter wavelengths (like violet). Suspended water droplets act like a prism: sunlight bends coming into each drop, then some of it bounces off the back of the drop, and bends again coming back out. The red bends more than the violet, the yellow in-between, giving us the effect of a spray of colors stretched across the sky: ROYGBIV.

Why is the rainbow is only a pale reflection of the sun, instead of a glare like a glimpse of the sun in a polished mirror? For the same reason we catch only a ghostly, pale reflection in a window at night: when light passes through a thin clear layer, most of it goes out the other side, and only a little is reflected back. The reflected amount varies between zero and sixteen percent and achieves a maximum that depends on the thickness of the layer: specifically, what light wavelength it is a multiple of. Any particular thickness of layer will favor a particular associated color. A layer with a constantly varying thickness, like a sheen of oil on top of water, will shows swirls of color. We call this effect "iridescence," from Iris, the goddess of rainbows, the messenger of the gods. (The plural of "iris" is "irides"; this root also explains the spelling and is the only way I can remember not to give it a double "r" as in "irradiation.") We see iridescence in soap bubbles, oil slicks, and some kinds of crystals.

Many living creatures also have learned the iridescence trick, though strangely it's more common in reptiles, fish, birds, and insects than in mammals. An exception is the golden mole, a varmint that predates the entire dinosaur era and has several archaic properties such as a cloaca (a combined port for liquid and solid elimination). Its fur has a golden sheen that is said to be a function of iridescence. It "swims" through loose sand like a Dune sandworm, which I find charming, but it is iridescence I started out to discuss here, so I'll try to get back on topic.

The Blue Morpho butterfly has iridescent wings, as do the scarab beetle and many birds, including the peacock and the hummingbird. Insects' effects usually result from thin, clear scales, while birds employ tiny periodic nanostructures in their feathers' hairlike "barbules."

Cuttlefish (below left) use a combination of pigment and iridescence to exhibit color, which they can change instantly, like a TV screen.

And of course, there are the beautiful nacres, formed from thin clear layers of shell.