The Caribbean and Cool Hand Lukeʼs Eyes
The blue sky. The deep blue sea. Paul Newmanʼs eyes. Bluebirds.
What these have in common is that none are (or were) blue. The sky, the ocean, blue irises in humans, and all blue bird feathers are, in fact, colorless. They appear blue because their physical structures alter light, making the blue end of the light spectrum visible, while absorbing or diffusing the other colors of light. Scientists call this ʻstructural color.ʻ
“Colors are the deeds and sufferings of light.”
- Johann Wolfgang von Goethe
If you remember high school science you remember that the visible light spectrum appears to us as white light, although itʼs actually composed of many wavelengths, frequencies, and energy levels, each one producing a different color of light. A prism – or water droplets in the case of a rainbow – breaks down light into itʼs many visible wavelengths.
As light – sunlight – passes through our atmosphere the blue wavelengths gets bounced around by the the molecules of oxygen and nitrogen, while the other colors of light, because of their different frequencies and wavelengths, pass through the atmosphere relatively undisturbed. The diffused blue light in the atmosphere is visible to our eyes, while the unscattered wavelengths remain invisible. The result: our colorless atmosphere appears blue. Scientists call this Rayleigh Scattering. Rayleigh scattering by water molecules is slightly different from the scattering by atmospheric gas molecules. Water, like the atmosphere, is colorless, but itʼs an excellent light ﬁlter, letting blue light waves penetrate and scatter, while the other wavelengths are simply absorbed without scattering. We see the scattered blue and not the absorbed wavelengths, so we see a blue ocean. Add the reﬂection of a clear blue sky and a white sand ocean ﬂoor, and you have the impossibly blue water of the Caribbean.
A person with brown or black irises has the pigment melanin in their iris. Without melanin, irises are colorless. However, they act, molecularly, something like the atmosphere; the red spectrum wavelengths penetrate to the back of the iris (where they are absorbed), while the blue wavelengths are scattered and reﬂected back. Someone with just a little bit of melanin in their irises has grey or green eyes, depending on the pigmentation of the melanin and amount.
And ﬁnally, bluebird feathers. Like water molecules, bird feathers appear blue because their cell structure absorbs the other wavelengths of light and reﬂect and scatter the blue. Different species of birds with blue feathers evolved with slightly different feather cell structures, producing different shades of blue. Also, like the human iris, some blue feathers contain small amounts of pigment – which is why a bluebird and a blue jay arenʼt the same color of blue.
The blue paint in a painting, and blueberries, on the other hand, are actually blue. Both contain blue pigment – although that doesnʼt make their color any less mysterious.
This blue planet we share is an amazing place to call home.