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I like the Mountain Bluebird. Four days ago, I watched it fly against the blue sky. What is striking about this is that neither the bluebird, nor the sky gains its colour from a pigment.<\/p>\n
Now, the majority of naturally coloured things around us show their colour as a result of pigments. A pigment is a chemical that absorbs only certain colours and reflects the others. So, a green object is green because chemicals absorb the other colours, but reflect green light. This behaviour is not true of the blue sky, and it is not true of the bluebird. Neither of them have a pigment that absorbs all the wavelengths other than blue. They produce blue by their geometric structure.<\/p>\n
Indeed, a blue chemical pigment is a rather rare thing in nature. Look around in the natural world. There are very few things that are blue: the clear sky, lake water, some berries, and a few birds. An explanation for the lack of blue pigments is unclear, but the consequences of the substitutional geometric structures are stunning.<\/p>\n
Indeed, birders know that pigmented colours are not the only way colours can be produced. Consider the gorget of a hummingbird. It may appear black as seen one way, and incandescent orange if the bird’s head is turned.<\/p>\n
In the early nineteenth century, one of the great unknowns was why the clear sky was blue. This was solved in the latter part of the century as being an interaction of light and the air molecules. Blue light has the shortest wavelength of visible light and thus is most easily scattered by an interaction with tiny molecules. The other colours had longer wavelengths and to a larger extent they just washed by molecules without having much interaction. The air molecules had no pigments; but the small blue wavelength was scattered whenever it interacted with the smaller molecules in the air. It was a blue colour resulting from a geometric structure.\u00a0<\/p>\n
But what about the bluebird? Now many birds come in a stunning array of colours. Most of them have pigment chemicals that colour parts of them, say, with red, yellow, or green. But, not bluebirds. They get the blue colour from the geometric structure of the feathers. This structural dependance becomes evident when feathers are ground up. Grind up a green, yellow or red feather and the resulting powder has the same colour. Grind a brilliant blue feather and the result is not blue but just a dark powder. Grinding has destroyed the geometric structure that produced the blue light.<\/p>\n
The blue of a bluebird results from tiny air cavities in its feathers. These cavities act like tiny particles by selectively scattering blue light, and as such, they behave analogously to air molecules creating the clear blue sky.<\/p>\n
A Mountain Bluebird flies against a blue sky. Both bird and sky are blue as a result of light scattered by very small geometric structures, rather than by a chemical pigment. Of course the branches of the red osier dogwood, also seen here, gains its colour from a pigment.
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I like the Mountain Bluebird. Four days ago, I watched it fly against the blue sky. What is striking about this is that neither the bluebird, nor the sky gains its colour from a pigment. Now, the majority of … Continue reading