The sky is blue because of Rayleigh scattering.

Rayleigh scattering happens when the sun’s light enters the earth’s atmosphere and the photons scatter.

The sun’s light races through space in a straight line. There is no atmosphere in space, therefore nothing to disrupt the movement or direction of the light.

Once those beams of light interact with the earth’s atmosphere, they encounter lots of tiny gas molecules, water vapor and other particles. Also, Earth’s atmosphere is roughly 300 miles thick.

Each beam of light is full of all colors, but when it bumps into these molecules, like nitrogen and oxygen, the light scatters and all the visible light moves around in all directions. Blues and purple have the shortest wavelengths which allow it to travel through all that atmosphere. Blue wavelengths are 4 times stronger than red.

The blue wavelengths are really the only wavelengths that can make it all the way from the top of the atmosphere down to our eyes. Our eyes then interpret those short wavelengths as blue.

As you get closer to the horizon, the blue is lighter and lighter, because the beam of light has more atmosphere to travel through, therefore diffusing the bright blue that you would see straight overhead.

Also, Rayleigh scattering is a type of particle physics. So, yeah, you just learned about particle physics too! Science is cool!