Question: why is the sky blue

Robert Lees answered on 10 Nov 2015:

This is such a fun question to think about, but also surprisingly complicated!

Visible light that you can see can be split up in to individual ‘wavelengths’, we see between 400 and 700 nanometre wavelengths. 450 is blue, 700 is red and in between is all of the other natural colours!

When the sun shines it’s light at the sky, the lower wavelength light has higher energy and interacts with more things in the atmosphere than the higher wavelength light – called Raleigh scattering (after the guy that discovered it).

So, the BLUE light (LOW wavelength) is bounced around more and gets reflected to our eyes from those molecules in the sky. The rest of the visible light doesn’t interact as much with those molecules and continues on it’s way, never being reflected in to our eyes.

The sky is RED when the sun light has already travelled really far through the atmosphere (most of the BLUE light has bounced off molecules in the sky already) and this happens when the sun is low in the sky.