Dear Jatin, I tried to answer both of these question in chapter 3 of http://xafs.org/Tutorials?action=AttachFile&do=view&target=Newville_xas_fundamentals.pdf Does the explanation there help? If not, do you have any suggestions for making it clearer? The basic physics is that an absorbed x-ray creates a photo-electron which will scatter from its neighbors. A portion of the scattered portion (so, a small amount) of that photo-electron will return to the absorbing atom. But Pauli's exclusion principle tells us that if an electron is at the absorbing atom, and has the same energy (and other quantum numbers) as the outgoing photo-electron, the outgoing photo-electron cannot be created in the first place. Thus, the scattering of the photo-electron from the neighbors affects whether the photo-electron can be created, and so whether the x-ray will be absorbed. The effect can be either destructive or constructive, as with nearly any wave-like phenomenon. It does seem a bit circular that the photo-electron scattering affects the existence of the photo-electron, but quantum mechanics leads to many much stranger behaviors. --Matt