a - p r, b - q s, c - p, d - q Characteristic X - rays are produced due to transition of electrons from oneenergy level to another. Similarly, the lines in the hydrogen spectrum are obtained due to transition ofelectrons from one energy level to another. In photoelectric effect electrons from the metal surface are emitted out upon theincidence of light of appropriate frequency. In β - decay, electrons are emitted from the nucleus of an atom. Moseley gavelaw which related frequency of emitted X - rays with the atomic number of thetarget material as √v = a ( z - b) . In photoelectric effect, energy of photons ofincident ray gets converted into kinetic energy of emitted electrons.