The number of spectral lines in a hydrogen atom is . . . . . . .

The wavelength of the first spectral line in the Balmer series of a hydrogen atom is $6561 \mathring A$. The wavelength of the second spectral line in the Balmer series of a singly-ionized helium atom is

If an electron in a hydrogen atom jumps from the $3^{rd}$ orbit to the $2^{nd}$ orbit,it emits a photon of wavelength $\lambda$. When it jumps from the $4^{th}$ orbit to the $3^{rd}$ orbit,the corresponding wavelength of the photon will be

In the given figure,the energy levels of a hydrogen atom have been shown along with some transitions marked $A, B, C, D$ and $E$. The transitions $A, B$ and $C$ respectively represent:

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

The ratio of the longest to shortest wavelengths in the Brackett series of hydrogen spectra is