Question

The size of molecules is than the wavelength of visible light

Answer 1

Molecules are the smallest scatterers, being about a factor of 1,000 smaller than the wavelength of visible light.

Answer 2

Answer:

Keep in mind that light is an EM wave, which means it has an oscillating field. There is actually dipole formation, but we can explain it using a very simple model. (classical mechanics)  

Now suppose a photon of light (using wave-particle duality here because its easier to visualise) which is oscillating bashes into a gas atom in the air. What happens next looks like this:

The particle first absorbs the energy of the vibrating photon. In doing so, it raises its own energy and starts vibrating itself.  to come back to a stable state, the particle must get rid of this energy somehow. It does so by emitting the excess energy in the form of radiation. The radiation emitted in all directions is equivalent to the energy of the incident photon (or the wavelength of light) - which is blue light in this case. In the end you have blue light being scattered everywhere, in all directions. :P

Hope it helps you buddy