According to Rydberg equation, $\frac{1}{\lambda}$ = R $\left(\frac{1}{n_1^2} - \frac{1}{n_2^2}\right)$ Here, $n_1$ = 2, $n_2$ = 4 and R = 109677 $\mathrm{cm}^{-1}$ $\frac{1}{\lambda}$ = 109677 $\left(\frac{1}{2^2} - \frac{1}{4^2}\right)$ = 009677 $\left(\frac{1}{4} - \frac{1}{16}\right)$ = 009677 $\frac{4-1}{16}$ = 109677 × $\frac{3}{16} \mathrm{cm}^{-1}$ ⇒ λ = $\frac{16}{109677 \times 3}$ = $\frac{16}{219031}$ = 4.86 × $10^{-5}$ cm = 486 × $10^{-9}$ m = 486 nm [Since 1 nm = $10^{-9}$ m]