1. The problem statement, all variables and given/known data The X-ray spectrum of molybdenum has a particular characteristic spectral line of wavelength 6.6 *10^–11 m. The ionisation energy of molybdenum is 20 keV. Deduce that the energy of an X-ray photon of wavelength 6.6*10^–11 m is 19 keV. 2. Relevant equations E=h*f E=(h*c)/l h=planks constant c=speed of light l=lambda(wavelength) 3. The attempt at a solution Normally this is quite easy. You just plug in wavelength into the equation and it should give you the answer. However, the part that I do not understand is that in the markscheme the proper formula to use is E(ev)=(h*c)/(l*e). Were e is elementary charge. Now this does not make sense to me as (h*c)/(l*e)=V (Voltage). So could someone explain why I need to use elementary charge.