Red light , how ever bright it is , cannot produce the emission of electrons from a clean zinc surface , But even weak ultra violet radiation can do so why ?

Question

$\begin{array}{1 1} \text{Frequency of U.V radiation is lesser than the threshold frequency } \\ \text{Frequency red light is greater than the threshold frequency } \\ \text{Frequency of u. v radiation is greater than the threshold frequency } \\ \text{Frequency of red light is greater than the threshold frequency } \end{array}$