In a historical experiment to determine Planck's constant, a metal surface was irradiated with light of different wavelengths. The emitted photoelectron energies were measured by applying a stopping potential. The relevant data for the wavelength $(\lambda)$ of incident light and the corresponding stopping potential $\left(V_{0}\right)$ are given below : \begin{center} \begin{tabular}{cc} \hline $\lambda(\mu \mathrm{m})$ & $V_{0}($ Volt $)$ \\ \hline 0.3 & 2.0 \\ 0.4 & 1.0 \\ 0.5 & 0.4 \\ \hline \end{tabular} \end{center} Given that $c=3 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1}$ and $e=1.6 \times 10^{-19} \mathrm{C}$, Planck's constant (in units of $J \mathrm{~s}$ ) found from such an experiment is