I'm a computer scientist who was recently given a book about quantum physics. I am confused about something in the book. Imagine shining an x-ray (produced by copper K alpha 1 emission) on an aluminium foil. I understand the basics of photoelectric effect. You shine the x-ray on target aluminium atoms, the K-atom is ejected, creating a hole, and then an L-atom drops down and in the process an X-ray is produced. In the book, the energy of the X-ray (produced when the electron from L-shell drops down and occupies the hole) is given as 8047 eV (8.047 KeV). What I don't understand is, this is the difference in the binding energy of COPPER. But the ejected electron is ejected from the ALUMINIUM foil. Surely the difference in binding energy should be the one in the aluminium foil, not the difference in the incident X-ray?