\(\lambda=\frac{h c}{E_{g}}\)

where \(\mathrm{E}_{\mathrm{g}}=\) energy gap of semiconductor

\(=1.9\, \mathrm{eV}\)

\(=1.9 \times 1.6 \times 10^{-19} \,\mathrm{V}\)

\(\lambda=\frac{6.6 \times 10^{-34} \times 3 \times 10^{8}}{1.9 \times 1.6 \times 10^{-19}} \,\mathrm{m}\)

\(=6.5 \times 10^{-7}\, \mathrm{m}\)

\(=650 \times 10^{-9}\, \mathrm{m}\)

\(=650 \,\mathrm{nm}\)