The relation between current and applied voltage illustrates the nature of the photoelectric effect. For discussion, a light source illuminates a plate P, and another plate electrode Q collects any emitted electrons. We vary the potential between P and Q and measure the current flowing in the external circuit between the two plates.
If the frequency and the intensity of the incident radiation are fixed, the photoelectric current increases gradually with an increase in the positive potential on the collector electrode until all the photoelectrons emitted are collected. The photoelectric current attains a saturation value and does not increase further for any increase in the positive potential. The saturation current increases with the increase of the light intensity. It also increases with greater frequencies due to a greater probability of electron emission when collisions happen with higher energy photons.
If we apply a negative potential to the collector plate Q with respect to the plate P and gradually increase it, the photoelectric current decreases, becoming zero at a certain negative potential. The negative potential on the collector at which the photoelectric current becomes zero is called the stopping potential or cut off potential[13]
i. For a given frequency of incident radiation, the stopping potential is independent of its intensity.
ii. For a given frequency of incident radiation, the stopping potential is determined by the maximum kinetic energy K_maxof the photoelectrons that are emitted. If qe is the charge on the electron and V_0 is the stopping potential, then the work done by the retarding potential in stopping the electron is q_eV_0, so we have
q_eV_0 = K_max.
Recalling
K_max = h left(f - f_0
ight),
we see that the stopping voltage varies linearly with frequency of light, but depends on the type of material. For any particular material, there is a threshold frequency that must be exceeded, independent of light intensity, to observe any electron emission.