Explain: "For photoelectric emission, the frequency of incident radiation should be greater than the threshold frequency."

(N/A) The threshold frequency $(\nu_0)$ is the minimum frequency of incident radiation required to eject electrons from a metal surface.
Each metal has a unique work function $(\Phi = h\nu_0)$, which determines its specific threshold frequency.
If the frequency of incident radiation $(\nu)$ is less than the threshold frequency $(\nu < \nu_0)$, the energy of the incident photons is insufficient to overcome the work function of the metal, and no photoelectric emission occurs, regardless of the intensity of the light.
If $\nu > \nu_0$, the energy of the incident photons is greater than the work function, and photoelectric emission occurs instantaneously (within $10^{-9} \, s$ or less), even if the intensity of the radiation is very low.