The frequency of revolution of an electron in the $n^{\text{th}}$ orbit of a hydrogen atom is

What is the change in angular momentum of an electron in a hydrogen atom when it transitions from the $4^{th}$ orbit to the $5^{th}$ orbit? $(h = 6.6 \times 10^{-34} \, J \cdot s)$

Line spectrum was first of all theoretically explained by

The frequency of revolution of an electron revolving in the $n^{th}$ orbit of an $H$-atom is proportional to:

$A$ particular hydrogen-like ion emits radiation of frequency $3 \times 10^{15} \,Hz$ when it makes a transition from $n=2$ to $n=1$. The frequency of radiation emitted in a transition from $n=3$ to $n=1$ is $\frac{x}{9} \times 10^{15} \,Hz$, where $X = \text{ . . . . . . }$.

The ratio of the area of the first excited state to the ground state orbit of a hydrogen atom is: