Why is the sky blue?

As an Exploding start for my blog, the question that has bothered many of us since our childhoods: why is the sky blue?
[Really lazy people can skip the basics and jump ahead to after the second picture! They will still get the general idea I think.]
[Long post, but worth it! :) ]

Basics: Visible light - everything you see - is actually an electromagnetic wave (EMW). One way to imagine light is constantly vibrating electric and magnetic fields, just like waves.

So as a wave, it has a few important properties, like wavelength (the vibration of the wave in space) and frequency (the vibration of the wave in time). [*Anyone a bit more into physics can see that these two are basically the same if we consider space-time;] These two are interconnected by a constant (the wave speed = the speed of light), so if we know for example the frequency then we immediately know the wavelength. If the wavelength is small then the frequency will be large and vice verse. This can be expressed in a simple formula:

speed (which is constant)=wavelength X frequency

For the rest of the post I will be talking only about the frequency and not the wavelength.

Summary on the basics: Light is an electromagnetic wave; important property: frequency.

Ok, what changes when we change the frequency of light? Everything! What will be important for us: the color. Check it out:

As you can see, visible light is only a small part of the ElectroMagnetic Spectrum. The different colors of visible light are also different frequencies of light - blue has a higher frequency and red has a lower frequency.

Why is all this important, when all we want to talk about is the sky?

The key: When light falls through a medium it scatters, and different frequencies scatter in different amounts.

The medium in our case is air. What is scattering? Imagine someone shining a green laser pointer through the air - you don't see the beam, only the beginning and the end of it. Now imagine someone pointing a laser through smoke - now you see the whole beam: this is exactly the scattering of light. Light from the laser would originally travel straight through the air and none would get to you eyes, but because of the smoke some of it scatters and you see some green from where it passes through.

Air? Exactly the same! But the scattering is much weaker than with the smoke and laser story. The light source is the sun (mostly a homogeneous mix of all the frequencies of light (=white light)).

How does light scatter? Higher frequencies scatter more than lower frequencies. How much more? Much more! For the geeks: the amount of scattering is proportional to the fourth power of the frequency!!

This means low frequencies barely scatter and high frequencies scatter a lot!! So when a beam of sunshine comes through the atmosphere, the bluer, higher frequency parts tend to go off into other directions and the lower frequency reds go straight through, which means:

If you look directly at the sun, it looks reddish, if you look anywhere else, it looks blue!

Of course if the blue light scatters out of every single beam of light then only the redder parts remain.

Topics in the follow-up post: how to get a sunburn while freezing to death; why baby fish are asking their parents the same questions as we are and what to talk about with your girlfriend/boyfriend at a romantic sunset.