## Physics Questions People Ask Fermilab

You wrote:

Why is the sky blue? Does it have something to do with the Cerenkov effect?

Mr. Knight,

The reason that the sky is blue is due to the scattering of light by the atmosphere. It is not due to the Cerenkov effect. I discuss both effects below.

The light from the sun is all colors. You can verify this by taking light from the sun and putting it through a prism. You see all of the colors of the rainbow.

These colors are actually different wavelengths of light. Red light has a wavelength of about 700 nanometers, blue light is about 400 nanometers. The molecules of air (particularly nitrogen and oxygen) preferentially scatter blue light over red light. Thus, if you were to look directly at the sun with and without the earth's atmosphere, you would see that the sun is a little redder (i.e. less blue) when the atmosphere is there.

So where does the blue light go? It is scattered in all directions. Consider how you actually see. Light goes from the glowing object and hits your eye. Your eye can distinguish the direction the light comes from. However, while it is obvious that the light from the sun makes the sky blue, the blue sky light comes from many directions. So what is really happening is that light that would not normally hit your eye (say light from the sun that is skimming through the atmosphere at a very high altitude....for instance where jet airliners fly) gets scattered by the air. It gets scattered in all directions, some fraction of it downwards, where the light can then hit your eye. And since blue light is scattered more than red, the scattered light that hits your eye is blue, and, voila! blue sky.

There is a useful explanation of this that you might want to investigate, which I append here.

Cerenkov light is quite a different beasty. It occurs when a particle that has a net electric charge travels through a material faster than light would travel through the same material. So, if the blueness of the sky were caused by Cerenkov light, it would have to be from charged particles from the sun. Cerenkov light is also typically more ultraviolet rather than blue, although Cerenkov light is emitted in many colors. It is a little more difficult to find some non-scientist web pages for this topic, but I recommend:

This is fairly technical, but there is a graph (the first one) which shows the Cerenkov spectrum for a couple of materials. I suggest you concentrate on the black solid and black dashed lines. You see that the bulk of the light occurs at wavelengths that we can't see. However, enough of it occurs in eye-friendly wavelengths that it can be seen. One case where you can see the Cerenkov easily (well relatively....) is to look at a pool of water which contains radioactive fuel from a nuclear reactor. The water in the pool seems to glow blue.

But this effect isn't the cause of the blue sky.

Good question though....

Dr. Fermilab....

Back to Questions About Physics Main Page