\(E_{g}=\frac{h c}{\lambda}\)
For visible region
\(\lambda=\left(4 \times 10^{-7}-7 \times 10^{-7}\right) \mathrm{m}\)
\({{E_g} = \frac{{6.6 \times {{10}^{ - 34}} \times 3 \times {{10}^8}}}{{7 \times {{10}^{ - 7}}}}}\)
\(=\frac{19.8 \times 10^{-26}}{7 \times 10^{-7}}\)
\(=\frac{2.8 \times 10^{-19}}{1.6 \times 10^{-19}}\)
\(E_{g}=1.75 \mathrm{eV}\)