The work function

 The work function, f, is defined as the least energy that must be supplied to remove a free electron from the surface of the metal, against the attractive forces of surrounding positive ions. Shown here is a photocell. When incident light of appropriate frequency strikes the metal (cathode), the light supplies energy to the electron. The energy needs to remove the electron from the surface is the work not all of the energy goes into work! As you can see the electron then move across the gap to the anode with a certain speed and kinetic energy.