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Out of the blue

There are many questions parents dread to hear from their children. They have the potential to embarrass, some because they involve topics seldom discussed among polite company, others because you feel you really should know the answer. Let me help you with one of the latter.

“Why is the sky blue?” The answer to this seemingly simple question also explains the blue of bluebirds, jays and many other birds with blue feathers. First, it is important to note that there are no blue pigments in bird feathers, or the sky either for that matter. There are brown pigments, red pigments, yellow pigments and others, but no blue pigments in feathers.

Pigments work by absorbing light. A red pigment absorbs all the colors of the rainbow but red. Red is the only color left to be reflected back to your eyes. If you take a red feather from a finch and grind it up, you will get a red powder. The pigment is still there absorbing and reflecting light as it did in the intact feather. However, if you grind up a blue feather from a jay, the blue disappears into a brown powder.

Colors can also be created without pigment. We call these “structural” colors. Structural colors involve tricks with light. One kind involves iridescence, a topic for another time. The blue of the sky and bluebird feathers is created in a still different manner.

I’ll bet if you went back and looked at your physical science notes from either middle or high school, you would find the term “Tyndall Effect” scribbled somewhere. This is the scattering of light due to very, very tiny particles. Particles of water in fog scatter a headlight beam. Fat droplets in milk scatter a light shown through it.

If all the particles are one size, then only one color is scattered. The other colors pass right on through.

Now to explain why the sky is blue. Unless you look directly at the sun, rays of light pass to your left and right and especially overhead. The molecules in the atmosphere are just the right size to scatter the color blue. If the molecules in the air were slightly larger, the sky might appear green or even red. If the molecules were smaller, maybe only ultraviolet light would be scattered, a color we can’t see, and the sky would appear black. It’s interesting to consider such a world where the sky is black and the clouds bright white. The water droplets in clouds come in all sizes and scatter all light.

The same sort of thing happens in a bluebird feather. Embedded within the feathers are particles just the right size to scatter blue light. The other colors of the rainbow pass through only to be absorbed by a layer of melanin deeper in the feather. Just like the sky, the blue — and only blue — is scattered to our eyes. The fine structure of a feather is truly impressive.

Here is something to ponder as you enjoy an azure blue scrub jay perched in the morning sun.

Stewart Janes is a biology professor at Southern Oregon University. He can be reached at janes@sou.edu.

ìWhy is the sky blue?î The answer also explains the blue of bluebirds, jays and many other birds with blue feathers.