In which of the following series does the $121.5 \ nm$ line of the spectrum of the hydrogen atom lie?

In the hydrogen spectrum,the shortest wavelengths of the Lyman and Balmer series are $\lambda_1$ and $\lambda_2$ respectively. The Rydberg constant of hydrogen is

In a hydrogen sample,if the atoms are excited to states with principal quantum number $n = 20$,then the number of different wavelengths which may be observed in the spectrum is:

The shortest wavelength in the Paschen series of the Hydrogen spectrum is (Rydberg constant of hydrogen $R_H = 1.097 \times 10^7 \ m^{-1}$). (in $nm$)

$A$ hydrogen atom absorbs radiation of wavelength $975 \, \mathring{A}$ and transitions from the ground state to an excited state. How many spectral lines are possible in the emission spectrum?

The ratio of wavelength of spectral lines $H_\alpha$ and $H_\beta$ in the Balmer series is $\frac{x}{20}$. The value of $x$ is $...............$