Why is our sky blue?

To answer the question: Why is our sky blue? we must first find out what exactly light is and what makes it up. 

Usually people talk about the amount of light which is emitted by a lamp (just an example). The light they are referring to in this case is usually white or yellow, but it is in fact made up of literally billions of different wavelengths. The human eye can only see light of approximately 400 to 700 nanometres, this is called the visible light and it stretches from purple (<400 nm) all the way to red (700> nm). Just to compare, the entire light spectrum reaches from 10^-14 m, which equals 0.01 picometre, to 10⁴ m, which is equal to 10 kilometre.

We only need to have knowledge about visible light to answer the question, so let’s forget about the other spectra. From now on the term ‘’light’’ will be used, meaning the visible spectrum.

A white beam of light is made up of all colours. When this white beam of light is directed into a glass prism, the glass will slightly bend the light beam. However because blue light has a smaller wavelength than yellow and red, blue light will bend just a tiny bit more than the red and yellow light, thus separating the single beam into all the different colours. The effect: a rainbow is being formed. In natural rainbows, the glass prism is substituted by drops of water and the light source is the sun. That’s the reason why rainbows only appear when the sun is shining and it is raining at the same time.

Now that we know that light can be split into different colours, there is only one more thing applicable to the situation in our sky. It's generally known that we have an atmosphere around us which protects us from space, however the atmosphere has another very important function: in fact it has the same function as the prism in the previous example. The light source, the sun, sends its light beams towards the Earth and they hit the Earth at such an angle so that the atmosphere bends the light just enough to make sure the light blue light ( get it ;) ) is the light which hits the surface. The same magical event happens when the sun sets and the sky partly turns green, yellow and red; only this time the beams hit the atmosphere from another angle so the green, yellow and red colours hit the Earth's surface instead of the blue.

Fun fact: the (Dutch) handbook for natural sciences and mathematics describes a metre as the distance light travels in 1/299792458 second in a vacuum space. At first this seems like some random number but it actually corresponds with the speed of light, which is almost 300 million meters per second.