Physicist: The blue of the sky is sunlight that has been scattered by the air in a process called Rayleigh scattering. The probability that a photon of frequency is scattered is proportional to . So purple (the highest frequency we can see) gets scattered about 15 times as much as red (the lowest). So if purple is the most scattered color, then why is the sky blue? These two effects mean that both lower frequencies and very high frequencies won’t be seen in the color of the sky, and in between is a surprisingly sharp “sky blue”.