The Photoelectric Effect has a critical frequency dependency:

1. Threshold (Cut-off) Frequency ($f_0$): This is the minimum frequency of incident radiation below which no electrons are emitted, regardless of how intense the light is.
2. Requirement: The energy of a single photon ($hf$) must be at least equal to the Work Function ($\Phi$) of the metal. $hf\ge \Phi ⟹f\ge \frac{\Phi }{h}=f_0$ If $f<f_0$, the photons don't have enough energy to libearte an electron from the metal's surface.