What is the angular momentum of an electron in the fourth orbit of a hydrogen atom?

The wavenumber of the first spectral line of the Lyman series of $He^{+}$ ion is $x \ m^{-1}$. What is the wavenumber (in $m^{-1}$) of the second spectral line of the Balmer series of $Li^{2+}$ ion?

When an electron in a hydrogen spectrum transitions from the $7^{th}$ energy level to the $1^{st}$ energy level,what is the total number of spectral lines produced?

$A$ photon of wavelength $4 \times 10^{-7} \, m$ strikes on a metal surface. The work function of the metal is $2.13 \, eV$. Calculate:
$(i)$ The energy of the photon in $eV$.
$(ii)$ The kinetic energy of the emission in $eV$.
$(iii)$ The velocity of the photoelectron in $ms^{-1}$ ($1 \, eV = 1.6020 \times 10^{-19} \, J$,mass of electron $m = 9.10939 \times 10^{-31} \, kg$).

Why does the spectrum of an atom contain many lines even if only one electron is excited?

What is the hydrogen spectrum? Give its characteristics.