$${\lambda = \frac{hf}{E_g} = \frac{(6,63 \times 10^{-34}\, \mathrm{J \cdot s})(3,00 \times 10^8\, \mathrm{m/s})}{(1,9\, \mathrm{eV})(1,60 \times 10^{-19}\, \mathrm{J/eV})} \\ = 6,5 \times 10^{-7}\, \mathrm{m} = \underline{\underline{650\, \mathrm{nm}}}}$$