Photoelectric current increases with increase in accelerating (positive) potential. At some stage, for a certain positive potential of plate A, the photoelectric current becomes maximum or saturates. If potential of plate A is further increased, the photocurrent remains same. This maximum value of the photoelectric current is called saturation current. When the potential of the collector plate is made more and more negative (retarding) with respect to the plate emitter, the electrons are repelled and only the most energetic electrons reach the collector. The photocurrent decreases rapidly until it drops to zero at a certain sharply defined, critical value of the negative potential V0. For a particular frequency of incident radiation, the minimum negative (retarding) potential V0 given to the collector plate for which the photoelectrons are completely stopped from reaching collector or photocurrent becomes zero is called the cut-off or stopping potential.
