\(\sqrt{f}=K_\alpha(Z-1)\)
\(\sqrt{f}=(40)\)
\(\sqrt{f}=\sqrt{2.48 \times 10^{15}} \times 40\)
\(f=2.48 \times 10^{15} \times 1600\)
\(\frac{c}{\lambda}=2.48 \times 10^{15} \times 1600\)
\(\lambda=0.76 \times 10^{-10} \quad\) or \(\quad \lambda=0.76 \,\mathring A\)