Jason E. Andersen

First, a correction.

A few months back I wrote about how blue birds are not really blue because blue feathers aren’t really blue. In that post I said “light passes through the feather and, through the physics magic of refraction, the blue color is what we see.” Well, today I learned that was subtly wrong. It’s not the magic of refraction that causes the blue illusion at all. The structure of the feather is such that all but blue light is absorbed by the feather, reflecting only blue light. This is quite different from refraction.

Bonus Fun Fact: The sky, unlike blue feathers, is blue because of refraction.

As light passes through a medium, like air or water, the light is put slightly off kilter from its original trajectory. It is deflected by the thing it is passing through. This is the process of refraction. How much deflection depends on the light’s frequency. Blue light is higher frequency than green or red light, and so blue is deflected moreso. When sunlight hits the Earth’s atmosphere, the blue light is scattered more than the other types of light, shooting every which way in the sky. Some of it makes it to our eyeballs and we perceive it as blue.

So, that is why the sky and feathers are blue. But are they, really?