(i) Two important features of Einstein's photo electric equation
(ii) Explanation of observations and finding value of work function of surface $\mathit{Q}$
(i) Maximum kinetic energy $\left({\mathit{K}}_{\max }\right)$, of emitted electrons, depends linearly on frequency of incident radiations

Existence of threshold frequency for the metal surface ${\mathit{\varphi }}_0=\mathit{h}{\mathit{v}}_0$
(Any other relevant feature)
(ii) Since no photoelectric emission takes place from $\mathrm{P}$ it means frequency of incident radiation $\left({10}^{15}\mathrm{Hz}\right)$ is less than its threshold frequency ${\left({\mathit{v}}_0\right)}_{\mathit{p}}$.
Photo emission takes place from $\mathit{Q}$ but kinetic energy of photoelectrons is zero. This implies that frequency of incident radiation is just equal to the threshold frequency of $\mathrm{Q}$.
For $\mathrm{Q}$,
work function
${\mathit{\varphi }}_0=\mathit{h}{\mathit{v}}_0$
$=\frac{6.6\times {10}^{-34}\times {10}^{15}}{1.6\times 10}\mathrm{eV}$
$=4.125\mathrm{eV}$