Final answer:
The threshold frequency in the photoelectric effect is the minimum frequency of incident light necessary to eject electrons from a material. Below this frequency, no electrons are ejected no matter how intense or long the light exposure is. The relationship between photon energy, threshold frequency, and electron ejection is described by the equation KE = hf - BE.
Step-by-step explanation:
The correct description of threshold frequency within the context of the photoelectric effect is: It is the minimum frequency of incident light required to eject electrons from a material. When light shines onto a metal surface, electrons can be ejected from the material; however, this only occurs if the light has a frequency above a certain minimum value known as the threshold frequency. If the incident light's frequency is below this threshold, no electrons will be ejected regardless of the light's intensity or duration of exposure.
Using the equation KE = hf - BE, where KE is the maximum kinetic energy of the photoelectron, hf is the photon's energy, and BE is the binding energy of the electron to the metal, we see that the threshold frequency (fo) is the frequency at which the photon's energy is just sufficient to overcome the binding energy. Photons with a frequency greater than the threshold frequency will eject electrons with kinetic energies that increase linearly with the frequency of the incident light. This linear relationship and the instantaneous nature of the photoelectric effect could not be explained by classical theories but were successfully explained by Einstein's proposal of the photon model of light.