Photoelectric emission is a fundamental quantum effect, responsible for turning light into the electrons that matter. Among its major features, startling enough, is the existence of a threshold frequency, which indicates the absence of a photoelectric emission process for a specified material. In this article, we will look at the threshold frequency, its importance, and the reason as to why it exists in detail.
The threshold frequency is the lowest frequency of incident radiation able to liberate electrons from the surface of a material by means of the photoelectric process. As long as the frequency of light is lower than this threshold, no electrons are emitted irrespective of the intensity of light.
The threshold frequency stems from the notion of the work function ( \phi ), which is defined as the minimum energy needed to free an electron from the surface of a material.
Light consists of energy in discrete packets known as photons, each with energy ( E = hf ). An electron on the surface of a substance can absorb the energy of one photon. If the photon’s energy is less than the work function, the electron does not have enough energy to overcome the binding forces of the material.
A material's work function depends on its electronic structure and surface properties. Sodium has a low work function (≈2.3 eV), while platinum has a high work function (≈5.6 eV), affecting their threshold frequencies.
Hertz and Hallwachs observed light-induced electron emission above certain frequencies. Einstein explained this using photon energy (ℎ𝑓 ≥ 𝜙), later validated by Millikan’s experiments, confirming the threshold frequency concept.