Structural Coloration: From Blue Jays to Butterflies, find out how colors can be based on nanostructures instead of…

Structural Coloration: From Blue Jays to Butterflies, find out how colors can be based on nanostructures instead of pigments. 

Originally shared by Tom Eigelsbach

Researchers are working to understand how often-colorless biological nanostructures give rise to some of the most spectacular technicolor displays in nature.

So what do the feathers of blue-jays and peacocks, the blue eyes of some gorgeous women, the opalescence of some gem stones, and the blue sky above us all have in common? Math. Because none of them are actually blue: rather, they are all colorless, yet they scatter blue light but not larger wavelengths of light. Rayleigh scattering in the sky, and Tyndall Scattering in blue feathers and eyes. How does this work, exactly? With a delightful mixture of microbiology and physics. 

The formulas have an inverse fourth power dependence on the wavelength of the light  (λ^−4), which in English means the red and blue ends of the spectrum get treated differently, with the blue light being scattered, unlike the red light. Which accounts for why our sky is blue and not red or yellow or white, for instance. You can also see Tyndall scattering if you have friends who ride motorcycles: look for blue smoke spewed from them; the particles are provided by burnt engine oil, and they also scatter only the blue light. 

But it’s much more unintuitive in feathers and eyes, because we know that pigments tend to cause coloring in animals, and they do for most other colors of feathers, for instance. But not for blue feathers; and not for the colors in the irises of our eyes. Blue feathers and eye colors are called “structural colors” because they are generated by light interacting with the 3-D shape of the colorless material, as opposed to a colored pigment or dye, which simply selectively absorbs some colors and reflects others. Note that pigments and dyes fade (think of your old blue-jeans or tie-dyed shirts that have faded). Structural colors, on the other hand, can easily last for hundreds or thousands of years or more in the feathers after the animal has died.

With blue feathers, blue-jays and other blue birds as well have no blue pigment at all of any kind in their feathers. Rather, blue-jay feathers consist of extremely small specialized cells. As the feather grows, keratin elongates in the cells and separates from water. Then when the cell dies, the water evaporates and the keratin forms permanent, honeycombed, sponge-like structures, called “barbs” in blue jays. These structures absorb the red end of the spectrum and scatter and reflect the blue end, thus making the blue-jay appear blue to us. 

In our eyes, the colored part, the iris, is composed of two cell layers: the front stroma layer; and the rear epithelium layer. The stroma consists of colorless collagen fibers. The stroma only occasionally will contain brown melanin pigmentation, but sometimes is totally colorless, containing no melanin. When this occurs, and the stroma is colorless, we have blue eyes, because when light enters the eyes the blue light gets scattered and reflected back out. This also accounts for the variations in the shades of color of someone’s blue or green eyes, depending on the lighting they are in.

Article on eyes: 

https://medium.com/@ptvan/structural-eye-color-is-amazing-24f47723bf9a 

Article on Rayleigh scattering in the sky:

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

Article on the why the Martian sky is Pink, and not Blue:

http://www.doublexscience.org/everyday-science-why-is-the-sky-pink/

Article on blue feathers below.

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