Calculation of wavelength of electron in first excited state
Radius of orbit
$\mathit{r}={\mathit{r}}_0{\mathit{n}}^2=0.53{\mathit{n}}^2\times \mathit{Å}$
$=0.53\times 4\mathit{Å}$
$=2.12\mathit{Å}$ $[\because$ Here $\mathit{n}=2]$
For an electron revolving in orbit, according to de Broglie relation


$2\times 3.14\times 2.12\times {10}^{-10}=2\mathit{\lambda }$
$\mathit{\lambda }=3.14\times 2.12\times {10}^{-10}$
$=6.67\mathit{Å}$