\(E=\frac{h c}{\lambda}=\frac{6.6 \times 10^{-34} \times 3 \times 10^{8}}{6840 \times 10^{-10}}\)
\(=\frac{6.6 \times 3 \times 10^{-26}}{6.84 \times 10^{-7}}\, \mathrm{J}\)
\(=\frac{6.6 \times 3 \times 10^{-26}}{6.84 \times 10^{-7} \times 1.6 \times 10^{-19}}\, \mathrm{eV}=1.8 \,\mathrm{eV}\)