Why Is the Sky Blue?

We should begin with the Sun itself, a big hydrogen-fusing ball of plasma with a surface temperature of about 5,780 K (5,507°C or 9,944°F). At this temperature, the Sun radiates strongest in the green part of the spectrum. 

It is at this point that we can come to understand a little more about the light that the Sun is producing: It peaks in the green part of the spectrum, but a very broad range of electromagnetic radiation (light) is being given off. The Sun gives off light in a variety of forms, from X-rays and UV radiation (which causes sunburn), all the way through to radio waves. When the electromagnetic radiation eventually hits the Earth, just about the only part that doesn’t reach the surface is the X-rays, as they’re blocked by the atmosphere.

Now we’ve established that the Sun really is green, but this doesn’t really explain why the sky is blue. Or does it?

We started off at the Sun, and the next step along this little journey is the Earth’s atmosphere, where all the excitement happens.

When light enter the atmosphere, it has to pass through all the molecules it encounters. The more energetic part of the spectrum (blue) is scattered more than the less energetic (red), meaning that the less energetic light is able to pass through relatively unscathed. When the blue light is scattered it is scattered in all directions, causing the entire sky to be blue.

You might now be asking why the sky isn’t violet. We can see violet, so it is not at all unreasonable to ask this. The reason is actually somewhat related to the way that we see the Sun as being yellow as opposed to being green. Violet is the wavelength of light that is scattered the most because it is more energetic than the blue light. Our eyes, however, distinguish colour by “seeing” in three colours: red, green, and blue. This means that, to be able to see violet, we have to mix two colours together: red and blue. Since red light isn’t scattered much, there isn’t enough red for our eyes to process. This is why we don’t see the violet sky.