As I understand it Rayligh scattering is used only when the particles that scatter light have diameters smaller than the wavelength of light, this seems to include atoms and molecules. I also know that Rayleigh scattering predicts that the intensity of the scattered light is inversely proportional to the wavelength to the fourth power.
I also know that Mie scattering is a generalization for all other sizes of particles and that if the size of the particle is much bigger than the wavelength of light the intensity is pretty much independent of the wavelength.
Every particle is ultimately composed of atoms and molecules... so why can't we use the Rayleigh approximation all the time?
I know that small water particles scatter light according to the Mie solution, and look white in clouds as a result... but water molecules, for instance at sea, scatter light in a way which can be described by Rayleigh scattering, this adds to my confusion!