Regarding absorption of light I have a question regarding absorption of light. When looking at the absorption spectra of for example "Chlorphyll a",  two absorptions peak can be seen in the visable spectra of light ( one at around 425 nm and one at 680 nm). I have been told that if a photon has suffient energy to excite and electron to a higher energy state (LUMO state) the material has absorbed the photon. This makes me a bit confused since if "Cholorphyll A" can absorb ligh at 680 nm ( red colour) why can't it absorb light at every other wavelength that has a higher photon energy? Surely all the other photons at lower wavelength (higher energy ) than at 680 nm must have suffient energy to excite the electron to higher energy state if photons at 680 nm can do it.
Clearly I am missing something here. 
Thank you in advance. 
 A: First we must clarify:


*

*if a photon interacts with an atom, three things can happen:


*

*elastic scattering, the photon keeps all its energy, but changes angle

*inelastic scattering, the photon gives part of its energy to the atom, and changes angle

*absorption, the photon gives all its energy to the atom, the valence electron absorbs it and moves to a higher energy level


*in the case of absorption, the energy level of the photon has to be exactly the same as the energy level of the difference between the current energy level of the valence electron and the next level

*it is also possible to knock off an electron from an atom (photoelectric effect).

*the reason why the energy level of the photon needs to be the same as the difference between the electrons energy levels, is that electrons around the nucleus can only be in quantized energy levels according to QM.

*there is no middle level of energy for an electron around the nucleus

*the energy levels at which electrons can orbit a nucleus (it is not orbiting like a planet), are defined by QM at specific levels
A: In order for the excitation to occur there must be resonance. The energy of the absorbed photon must match the energy of the transition. In this case the transition is between bound states with well defined energies.
