Einstein's photoelectric equation,
K.E. of photo electron = incident energy of photons - Work function

where v0 is called threshold frequency.
(i) Threshold Frequency : For a given metal, there exists a certain minimum frequency of the incident radiation below which no emission of photo electrons takes place. This frequency is called threshold frequency.
(ii) Stopping Potential : It is that minimum negative potential given to anode in a photocell for which the photoelectric current becomes zero. It is denoted by V0. It is independent of the intensity of the incident light.