I am trying to understand why radio waves pass though the human body, light does not, but X-rays still pass.
Are you aware of quantum mechanics? Light interacts with matter at the microscopic level where quantum mechanics reigns.
In my mind I imagine that radio waves don't supply enough energy and don't excite any interesting modes of the material that they hit. This is in contrast to the photon excitation and associated scattering I imagine when a light beam hits a material.
Quantum means "a specific amount" of energy. Each photon hitting matter has a specific amount of energy which, when completely absorbed, it has to give up as a "quantum", all together. Radio wave photons do not find corresponding exact energy receptors when they hit most materials, except metals, where they do, that is how we can have shielding and bad reception, when metal intervenes.
I am confused as to why this doesn't happen for X-rays? I mean they come out the other end, so I assume that they have a pretty good transmission though soft tissue.
Certainly, the 1MeV is a enough to excite the band-gaps in the human body which, in turn, should cause all the X-rays to be absorbed?
As I explained above, that one mev has to be absorbed in one scatter and that is nuclear energy levels not electronic levels in atoms. The nuclei are shielded by the electrons around them and are not seen by photons for that reason. Thus the majority of x-rays will pass through most materials if they are not too thick:
When the path in matter is long, other interactions of the photons can take place, like compton and raman scatterings which will degrade the energy of the photons, even of radio frequencies, and after long lengths attenuate the beams.
