4. The stopping voltage is independent of the intensity of the incident radiation.
These observations suggest that electromagnetic radiation is a form of energy that releases electrons from metallic surfaces and imparts to these electrons sufficient kinetic energy to cause them to travel to a negatively charged electrode. Furthermore, the number of photoelectrons released is proportional to the intensity of the incident beam.
The results of these experiments are shown in the plots of Figure 6-14, in which the maximum kinetic energy, or stopping energy eV0, of the photoelectrons is plotted against frequency for photocathode surfaces of potassium, sodium, and copper. Other surfaces give plots with identical slopes, h, but different intercepts, ω.