What kind of electromagnetic waves does a molecule or an atom absorb ro emmit?
Electrons orbiting the nucleus of an atom have particular ‘energy levels’ which are allowed. The difference(s) in these energy levels are always fixed, discrete amounts. If a photon of electromagnetic energy happens along and its energy (which is given by Plancks constant divided by the photons wavelength) is close to the value needed for an electron to change energy levels, the electron will absorb the photon and ‘jump’ to the higher energy level. The higher energy level may or may not be ‘stable’ and the electron may fall back to a lower energy level. When an electron falls to a lower energy level it emits a photon carrying exactly the amount of energy (as described above) that the electron has ‘lost’ falling to the lower energy level. (This is, in fact, exactly the principle upon which a laser is based).
