Define threshold frequency and on which factor does threshold frequency depend?

(N/A) The minimum frequency of incident radiation required to eject electrons from a metal surface is called the threshold frequency. It is denoted by $\nu_0$.
The value of the threshold frequency depends solely on the nature of the metal (the work function of the material).
For most metals,such as zinc,cadmium,and magnesium,the threshold frequencies lie in the ultraviolet region,which corresponds to short wavelengths.
For alkali metals,such as lithium,sodium,potassium,caesium,and rubidium,the threshold frequencies lie in the visible region.