Part 1: Blue Skys and Blue Feathers – The Scattered Blues

A while back I wrote about why the sky was blue and why some feathers are blue (here) – well I didn’t quite get it right, so I’m trying again. I’ve written a more detailed explanation which I’ll post in four parts.

When I look around, I see lush greens of temperate rain forest, rich browns of fertile soil, lively yellows in fluttering butterflies, and luscious reds in ripe berries – but, not a lot of blues. If the sky is clear, it’s the biggest blue object around, extending from horizon to horizon. Water reflects the blue of the sky, adding another layer of blue. On a lucky day, I’ll catch a glimpse of a Steller’s Jay showing off it’s blue and black plumage, or a shimmering silver-blue dragon fly will dart by. I might even see a rare blue flower. On a gray winter day, the blue eyes of my favorite companion may be the only brilliant blue around. Other natural places have their own blue components, but in general, blues aren’t common in nature. In fact, world-wide there just isn’t a lot of natural blue pigments, thus the blues we see are often the result of optical properties within an object. These colours created as the result of an object’s structure are called, creatively, ‘structural colours’. Blue is a very common structural colour, and to understand why we’ll need to start with some optics.

Sunlight is called ‘white light’ because it appears colourless. Within this colourless light lurks the full colour spectrum. Once, people thought white was the fundamental colour of light, and colours formed when something was added into the light. This theory was changed after the careful experimentation and observations of Sir Isaac Newton. Around 1670, Newton shone light through a prism creating a rainbow of hues on the other side. From this result, he concluded that white light contains all colours and that the prism simply separates them. Therefore, colour results from interactions between an object and light.

We now understand that white light is made up of tiny waves (which are simultaneously tiny particles if you want to add complexity). Light waves travel at the same speed but can have different wavelengths, that is, the distance between successive crests. Our brains perceive the different wavelengths as different colours. The longer wavelengths form reds, oranges and yellows, and the shorter wavelengths form greens, blues and violets. If you could watch waves of light pass by, more waves of blue would pass compared to waves of red – this means that the blue light has more energy. Light travels outward from its source, the sun, in a straight line until it collides with something. This collision could release all the hues in the spectrum or just a select few.

Scattering describes how light is diverted from its original straight path. Light scatters in two ways: coherent and incoherent. When scattering is coherent, spectacular effects such as iridescence can occur. Like a ball bouncing back from a flat wall, the light reflects precisely because the reflecting surface is geometrically regular. Similar colour light waves augment each other, further intensifying the effect. An iridescent feather’s colour can change depending on viewing angle, a phenomenon easily observed in a Anna’s Hummingbird gorget. Incoherent scattering resembles the result of throwing a rubber ball at a pole – it could bounce away in any direction. In this case, the scattering objects are randomly distributed relatively far apart. Scattering at one object occurs completely independently of the scattering at the other objects. Both coherent and incoherent scattering occur regularly in nature and can provide the mechanism for creating blue colours.

The photo is of a hyacinth macaw I took years ago at the San Diego Zoo.

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