The Energy Levels of the Electron

We're talking about transparency here today and what makes things see through. Daniel, you mentioned that it's kind of about how light can go into material, see the material, be near the material, but not interact with it. And you say it has something to do with the energy levels of the electron because you said that an electron floating out in space will always absorb an electron no matter what. How does that free space case work? If you zap an electron with a photon and the electron is out in free space, it means that there are no rules that govern the energy that the electrons can have.