The sun, from where we stand on the ground, is yellow. The sky is blue. So why is the sky around the sun white? Find out about the scattered light you never hear about, and why it means there is a 'halo' around the sun.

The sky above Earth is a beautiful radiant blue and we all know why: Rayleigh scattering. The tiny particles in our atmosphere interact with only the blue light coming from the sun. They scatter that light in every direction, and some of it travels downwards into our appreciative eyes. But Rayleigh scattering isn't the only game in town. It's initiated by particles much smaller than a wavelength of light. There are bigger particles in our atmosphere, especially on hazy days with a lot of water vapor coalescing into droplets.

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Particles larger than a wave of light also scatter light. The difference is, they aren't selective about which light they scatter. Red, yellow, blue, all the light that's out there they take in, and scatter around. Though perhaps "scatter" isn't the right word. Mie scattering, the type of scattering we're talking about, is highly directional. When a particle scatters light in this way, it tends to scatter it forward. Unlike the blue scattered light, which will travel in any direction, the white light will continue roughly on its original path.

Rayleigh scattering happens all over the sky, and because it sends out blue light in all directions, you can see it no matter where you stand. Mie scattering is also happening all over the sky, but it sends its white light only in one direction - the rough direction that the light was already traveling. You only see it if you happen to look in the direction the light was originally coming from - in other words, if you look near the sun. As you look nearer and nearer the sun, especially on hazy days, the blue light from Rayleigh scattering is overpowered by the white light from Mie scattering, and you see a white "halo" around the sun.

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[Via Mie Scattering.]