On a quantum level, when a photon encounters a material you have to ask whether the material is able to absorb a photon with that wavelength. When electrons are

On a quantum level, when a photon encounters a material you have to ask whether the material is able to absorb a photon with that wavelength. When electrons are bound to a specific atom (or are in specific molecular bonds) then there are only certain energy levels possible: that's one of the core features of quantum systems, and it's the reason that each material has its own characteristic "absorption spectrum" (or emission spectrum: same idea). Photons whose wavelength corresponds to an energy that doesn't very closely match what's necessary to raise an electron from one specific level to another will just pass on through. (Nuclei are in bound states with discrete energy levels, too, so they work the same way.) But one of the essential features of a metal is that the atoms all share a bunch of electrons that are free to move around more or less any way they'd like throughout the material. Because the electrons aren't trapped in one specific bound state, they have an essentially continuous range of energies available to them (just speed up or slow down a little to change your energy), so they are able to absorb photons of any wavelength at all. [Now, to actually understand why you get reflection rather than stopping with absorption would take me a little more work to figure out how to explain. My instinct keeps being to go back to the classical explanations at that point, but I wanted to focus on quantum here to address your question about electrons in orbit.]