Laws of photoelectric effect

Bringing together the results of sections 7.2.2 through 7.2.4, the detailed experimental study of the photoelectric effect can be distilled into five compact laws that any correct theory of the effect must explain:

1. For a given frequency of incident light, the number of photoelectrons emitted per second is directly proportional to the intensity of the incident light; correspondingly, the saturation current is also directly proportional to the intensity.
2. The maximum kinetic energy of the photoelectrons is independent of the intensity of the incident light.
3. The maximum kinetic energy of the photoelectrons from a given metal is directly proportional to the frequency of the incident light.
4. For a given surface, photoelectron emission takes place only if the frequency of the incident light is greater than a certain minimum value, the threshold frequency; below this frequency, no electrons are emitted no matter how intense the light.
5. There is no measurable time lag between the light striking the surface and the ejection of photoelectrons -- emission is essentially instantaneous. …