\(\frac{1}{\lambda}=R\left(\frac{1}{2^{2}}-\frac{1}{4^{2}}\right)=\frac{3 \mathrm{R}}{16}\) .... \((2)\)
Divide equation \((1)\) with \((\mathrm{2})\)
\(\frac{\lambda}{660}=\frac{5 \times 16}{36 \times 3}\)
\(\lambda=\frac{4400}{9}=488.88=488.9\, \mathrm{nm}\)