(i) Two important features of Einstein's photo electric equation (ii) Explanation of observations and finding value of work function of surface $Q$ (i) Maximum kinetic energy $(K_{\max})$, of emitted electrons, depends linearly on frequency of incident radiations $K_{\text{max}} = h v - h v_0$ Existence of threshold frequency for the metal surface $\phi_0 = h v_0$ (Any other relevant feature) (ii) Since no photoelectric emission takes place from $P$ it means frequency of incident radiation $(10^{15} \text{ Hz})$ is less than its threshold frequency $(v_0)_p$. Photo emission takes place from $Q$ but kinetic energy of photoelectrons is zero. This implies that frequency of incident radiation is just equal to the threshold frequency of $Q$. For $Q$, work function$\phi_0 = h v_0$$= \frac{6.6 \times 10^{-34} \times 10^{15}}{1.6 \times 10} \text{ eV}$$= 4.125 \text{ eV}$