### Rayleigh scattering in Cycles

Prompted by a question in a comment by Joakim Poromaa Helger I came up with this approach to emulate Rayleigh scattering (the effect that turns the sky blue and the sun red at the same time) in Cycles: by combining a transparent and a glass shader.

In the picture above there is a bright white sun on the right. In a material that exhibits Rayleigh scattering (like air) blue light is scattered more than red light. That means that light traveling from a lightsource through such material will be tinted red because blue light is scattered to all sides. In a direction away from the direct path of the light we see just this scattered blue light and not (or much less) the direct light.

Now if we combine a red(ish) transparent shader with a blue(ish) glass shader, the transparent shader will cause some of the light to be passed through the object without regard for index of refraction and give it a redish hue, while the glass shader will pass some (refracted) blue light through and reflect some. (In the sample picture I have turned off caustics because that takes forever to give a good quality render but also because in this setup caustics exhibit a blue color while it should be redish as well). The node setup looks like this:

With some extra trickery we can correct the color for any caustics as well:

(Note that this image took 10,000 samples and the caustics are still very noisy. Clearly caustics is not Cycles strong point. The light/shadow inside the orb are due to the fact I placed it slightly below the ground but am to lazy to fix it).

We achive the caustic coloring by arranging that the camera sees the blue glass shader while any object receiving diffuse light (like the floor) sees a red glass shader, a distinction we can make with help of the light path node:
Now obiously the effect here is exagerated for demonstration purposes and arguably it is not a real simulation of Rayleigh scattering but for artistic purposes this might be a good start.