Photoelectric emmision occurs only when the incident light has more than a certain minimum

4) For occurrence of photoelectric effect, the incident light should have frequency more than a certain minimum which is called the threshold frequency $\left({\mathrm{\nu }}_{\mathrm{o}}\right)$. 

We have, $\frac{1}{2}m{v}^2=h\nu -h{\nu }_o$ 

For photoelectric effect emission $\nu >v_o$where $\nu$ is the frequency of the incident light.