# Fermilab

## Inquiring Minds

 Questions About Physics

Color of the Sky

You asked:
I was wondering what color the sky is? I know that it looks blue but what is it actually?

Dear Nathalie,

This is really an excellent question with a difficult answer. I had a long discussion with one of my colleagues about this topic. I'm still not sure how best to explain the science, but let me give it a try.

First of all, think about all the things that can happen when light encounters an object. The light could be absorbed by the object. It might be reflected (like from a mirror) or diffusely scattered (like from most objects). Or, the light may be transmitted through the object (windows are a good example of this), and while the light is being transmitted, some of it may also be scattered in different directions.

The result (absorption, reflection, transmission, scattering) depends on the type of object and the wavelength of the light. In particular, for different wavelengths, the object might behave very differently. This explains how everyday objects (plants, clothes, Coke cans) have colors. These things absorb *almost* all the light that hits them, and reflect/scatter just certain combinations of wavelengths that our eyes then perceive as colors.

So what is the real color of the sky? Depends on your point of view! If I take a glass jar filled with air, it would look pretty clear and colorless. In fact the gasses that make up our atmosphere (almost all nitrogen and oxygen) are classified as "colorless" in chemistry reference books. Air transmits visible light almost completely. But nothing is *completely* transparent. If you shine white light through many miles of air (like sunlight in our atmosphere), you will notice that the air tends to scatter the bluer wavelengths and transmit the redder wavelengths. That's why the sky appears blue; it's scattering the blue portions of the sunlight.

So perhaps I should sum this all up by saying that the sky really is blue, but the air is nearly colorless. Hopefully my explanation isn't too confusing. If this answer leaves you with more questions, please don't hesitate to ask them!

Cheers,

- Craig Wiegert

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