The lowest frequency of light which can emit photoelectrons from a material is called the ‘threshold frequency’ or ‘cut-off frequency’ of that material.

For photoelectrons to be emitted from a given metal plate, the frequency of light incident on the plate should be higher than the threshold frequency.

From the graph, we see that Ek = Av – B, where A is the slope of the line.

The minimum energy required for the emission of photoelectron from a metal is called the work function of that metal.