A light of wavelength $3000 \stackrel{o}{\text{A}}$ falls on a metal surface. Ejected ${\mathrm{e}}^{-}$ is further accelerated by a potential difference of $\mathrm{}2\mathrm{V}$ , then final K.E of the ${\mathrm{e}}^{-}$ is found to be $8\times 10^{-19}\mathrm{J}$ . If threshold energy for the metal surface is $\mathrm{\text{'}}{\mathrm{\text{ϕ}}}^{\mathrm{\text{'}}}\mathrm{e}\mathrm{V}$ . Then find the numerical value of $8\mathrm{\varphi }$