Lenard used metal surfaces that were first cleaned and then held under a vacuum so that the effect might be studied on the metal alone and not be affected by any surface contaminants or oxidation. What he found, surprisingly, was that brightness had no effect on the energy of the photoelectrons emitted. Electrons ejected from exposure to a very bright light had the same energy as those ejected from exposure to a very dim light of the same frequency . This result was completely unexpected. Given that it is possible to move electrons with light and given that the energy in a beam of light is related to its intensity, classical physics would predict that a more intense beam of light would eject electrons with greater energy than a less intense beam no matter what the frequency .
Electromagnetic radiation can eject electrons from the surface of a solid.
These electrons are called photoelectrons , although they are indistinguishable from ordinary electrons. This process is called the photoelectric effect . The frequency of the light source is proportional to the maximum kinetic energy of the photoelectrons ejected. When charge is given in elementary charges, the energy will be calculated in electron volts . The intensity of the light source is proportional to the number of photoelectrons ejected.