Do you know what happens to the free atom or molecule when it is exposed to certain frequency of light?

Light of certain frequency falls on that atom or molecule. This atom or molecule reponds to the light whenever the size of the atom or molecule is comparable to the wavelength of light. If this condition is satified, the atom absorbs light and vibrate. Due to these vibrations, the atom re-emits, certain fraction of absorbed energy in all directions with different intensities.