When light rays of a definite intensity and a definite frequency fall on the plate P₁, photoelectrons are emitted from P1. If the plate P2 is at a sufficient positive potential, then all these photoelectrons reach the plate P2 and a maximum photoelectric current flows in the circuit.
On increasing the positive potential of the plate P, there is no increase in the photoelectric current, because all the electrons emitted from the plate P1 are already reaching the plate P2.
But If we double the intensity of light falling on the plate P1 the photoelectric current is also doubled. This means that the photoelectric current or the rate of emission of photoelectrons is directly proportional to the intensity of the incident light.
A graph between photoelectric current and intensity of incident light will be a straight line.
Now, if we give plate P₂ a negative potential relative to plate P1, then the current immediately decreases, but does not become zero. This shows that the electrons emitted from P, have kinetic energy. On increasing the negative potential of P₂, the current falls rapidly and finally becomes zero. At this stage, even if we increase the intensity of light falling on P₁, the photoelectric current is not obtained. This shows that P₁ emits electrons of different energies. When the plate P2 is at a small negative potential (relative to P), it repels these electrons. Therefore, the electrons with small kinetic energy do not reach the plate P₂ (high-energy electrons still reach) and so the current decreases. As the negative potential of Py is increased, the number of electrons reaching P₂ goes on decreasing. Finally, at a certain negative potential even the electrons of highest energy are not able to reach the plate P₂. The photoelectric current now becomes zero. The negative potential of P2 (relative to P1) at which the photoelectric current becomes zero is called ‘stopping potential’ or ‘cut-off potential’. If the stopping potential be Vo, then the maximum kinetic energy E of the photoelectrons will be given by
E = eVo