Why the sky is blue

Learning by teaching is a powerful device. When you take the time to explain a concept to someone else you are forced to think about that concept from a different vantage point.

What examples can I think of to explain this?
What is the clearest way one should think about this concept?

It requires a greater understanding of a subject to explain it to someone else than it does to just know something.

My nieces are in/entering the "why" stage. You know what I mean, but here's an example:



You need to eat.
Why?
Because you haven't eaten all day. (and I'm freaking getting an ulcer thinking about you not eating and then you getting sick and me getting arrested for being a neglible prick)
Why?
Because you didn't want to eat earlier.
Why?
If I knew that, my job would be much easier.
Why?
Because I'm going to the store, stay with grandma.



Summer is here and as much as I dread it, there are some great aspects. Warm nights, cold beer, cheap golf, movie marathons while continuously cursing away the sun -- but I especially love our sunsets and I catch far more of them during the summer. Others will probably have a similar appreciation for the above and I put two and two together.

They are going to ask "why is the sky blue?". Maybe not today. Maybe not tomorrow. But soon.

I had a vague idea of why, something about the density of the atmosphere and the reflection/absorption of light, but I'm going to try to learn this and explain it here for my own edification and maybe yours too. :)


Why is the sky blue?

There are two main components at work here:

Light
Light is made up of various colors. A rainbow is an example of the various colors that come from the sun's rays.

Our atmosphere
The Atmosphere contains mainly nitrogen and oxygen. Argon, Carbon Dioxide, and water are also present in small amounts. The Atmosphere also contains particles such as dust, ashes, pollen, etc. The Atmosphere is more dense with these particles the lower you are.


Light travels in a straight line, unless it is absorbed or reflected by an object. A gas molecule, when hit by sunlight, will absorb some of the light that tries to pass through it. When a molecule absorbs light, it will give off the color that was absorbed. Blue happens to be a smaller wavelength color, so it is more easily trapped by these gas molecules thus, giving off the blue light.



So why does the sky look bluer above you than off to the side? That's because the blue light above you (that has already been absorbed and radiated) is traveling straight down to you through the least amount of debris in the atmosphere. But light that is coming from around you is traveling through more of the dense atmosphere and that blue light is getting scattered and distributed.


Why is the sunset red?



As the sun nears the horizon the light from the sun must pass through the densest part of the atmosphere. The smaller waves are essentially decimated, scattered into a million other directions. The longer wave lengths (red, orange) are leftover and that consists of most of the light reaching you.


And there you have it.

But why?

I'm going back to the store.