If a quantum of radiation with an energy greater than the work function E_o, and therefore a frequency greater than fo, falls on a surface an electron will be emitted with some kinetic energy and escape from the surface. The kinetic energy of the electron is then just the difference between the energy of the quantum and the work function of the metal. This can be expressed in the following equation - called Einstein's photoelectric equation.

If we put a collecting electrode in front of the emitting surface in a vacuum we can detect the photoelectrons as a small current.

where V is the potential difference between the emitting surface and the collecting electrode.
If V is increased so that no more electrons can reach the detector this value for the potential is called the stopping potential for that surface and radiation.