Solution:

$ \begin{aligned}\\ & \mathrm{E}_{\mathrm{g}}=1.424+1.266 \mathrm{x}+0266 \mathrm{x}^2 \\\\ & \mathrm{E}_{\mathrm{g}}=1.424+(1.266 \times 0.07)+0266 \mathrm{x}(0.07)^2\\ \end{aligned} $

Eg = 1.513 eV

$ \lambda=\frac{1.24}{\mathrm{E}_{\mathrm{g}}}\\ $

$ \begin{aligned}\\ & \lambda=\frac{1.24}{1.513} \\\\ & \lambda=0.819 \mu \mathrm{m}\\ \\ & \lambda=\mathbf{0 . 8 2} \mu \mathrm{m}\\ \end{aligned}\\ $