Air is about four-fifths nitrogen (N2) and one-fifth oxygen (O2), with small admixtures of various other gases. N2 and O2 each have a molecule that consists of two atoms.
Air is about four-fifths nitrogen (N2) and one-fifth oxygen (O2), with small admixtures of various other gases. N2 and O2 each have a molecule that consists of two atoms.
The important thing about a molecule, for us, is that it consists of parts with positive and negative electric charges. The atomic nuclei are positively charged, and the electrons that inhabit the rest of the space are negatively charged. Since light is an electromagnetic wave, ultimately all interactions with it have to do with electric charges.
In the case of N2 and O2, the positive charge and the negative charge have the same average position under normal circumstances. The "center of gravity" of both nuclei and electrons is right in the middle of the molecule. But if you put one of these molecules in an electric field, it will nudge the electrons just a little bit in one direction, and the nuclei just a litle bit in the other direction. The molecule becomes polarized; it develops an electric dipole. Such a molecule develops its own little electric field, which looks something like the magnetic field of a bar magnet.
If the dipole is made to oscillate-- that is, if the positive and negative charge wiggle back and forth, out of phase with each other-- then the molecule will emit light waves of its own. The frequency of the emitted light is the same as the frequency of the wiggle.
Since an incoming light wave itself contains oscillating electric fields, it can make the dipole oscillate in this way, and cause it to emit light, which has the same frequency as the incoming light.
You may well ask: Where does the energy come from? It comes from the incoming wave. The outgoing radiation destructively interferes with the incoming wave in the forward direction. The original wave is lessened in intensity, and new waves move out in all other directions, so that overall energy is conserved (this requirement is sometimes called the "optical theorem"). The net effect is that light energy that was moving in a straight line from the sun ends up traveling in some other direction.
This is how air molecules scatter light.