An electron makes a transition from orbit n = 4 to the orbit n = 2 of a hydrogen atom. The wave number of the emitted radiations (R = Rydberg's constant) will be

$\frac{16}{3R}$ $\frac{2R}{16}$ $\frac{3R}{16}$ $\frac{4R}{16}$

$\frac{3R}{16}$

Wave number $\frac{1}{\lambda}=R\left[\frac{1}{n_1^2}-\frac{1}{n_2^2}\right]=R\left[\frac{1}{4}-\frac{1}{16}\right]=\frac{3 R}{16}$