We had a post-doctoral candidate interviewing this week who did his dissertation on OTR generated by relativistic electrons crossing a solid/vacuum interface. Unfortunately for him, he clearly didn't understand transition radiation, but it piqued the interest of the rest of us who interviewed him. Can anyone explain it? Links to explanations? Wiki is useless, and I couldn't find any good links on Google. For example, the candidate indicated that each electron generates a small fraction of a photon, which is nonsense and tells me it must be a collective effect of many charged particles with an associated current density threshold - why, and what determines the threshold?