Define threshold frequency?
For a given Photosensitive material, there is a certain minimum cut off frequency called Threshold frequency, for which stopping potential is zero.
Higher the work function of the material, greater is the Threshold frequency. (As you are aware that stopping potential is the minimum value of potential below which photoelectric current becomes 0.
See the figure below of two different metals having different Threshold Frequency.
thtesh hold frequency is the minimum frequency required for photoelectric emmission to occur..below this value photoelectric emission is not possible...it refers to the minimum no of of election which can excite the electrons from metal....
from einstien law of photoelectric effect
here fo is threshhold frequency of metal and rest k is kinetic energy of ejected photoelectron and f is frequency of light emmited...........