The maximum kinetic energy of the emitted electrons is

,
where is the maximum kinetic energy of the emitted electrons from the metal surface, is the frequency of light, and is the work function of the metal.

Also, if we want to stop the emitted electrons, we should put some potential difference onto them. Thus, the previous equation becomes

,
where is the electric charge, in this problem, should be . In this equation, we can prove that the potential difference is a linear function of the frequency.