A photosensitive metallic surface has work function, h v_0 . If...

The solution to our problem consists in Einstein's photoelectric equation. Einstein's photoelectric equation can be written as $\frac{1}{2} m v^2=h v-\phi$ $\Rightarrow \frac{1}{2} m \times\left(4 \times 10^6\right)^2=2 h v_0-h v_0...(i)$ and $\frac{1}{2} m \times v^2=5 h v_0-h v_0...(ii)$ Dividing Eq. (ii) by Eq. (i), we get $\frac{v^2}{\left(4 \times 10^6\right)^2}=\frac{4 h v_0}{h v_0}$ $$\begin{array}{ll}\Rightarrow & v^2=4 \times 16 \times 10^{12} \\ \Rightarrow & v^2=64 \times 10^{12} \\ \therefore & v=8 \times 10^6 \mathrm{~m} / \mathrm{s}\end{array}$$ NOTE: The efficiency of photoelectric effect is less than $1 \%$.ie, number of photons less than $1 \%$ are capable of ejecting photoelectrons.