How many emission spectral lines are possible when a hydrogen atom is excited to the $n$th energy level?

Heat treatment of muscular pain involves radiation of wavelength of about $900 \ nm$. Which spectral line of $H$ atom is suitable for this purpose?
$[R_H = 1.097 \times 10^7 \ m^{-1}, \ h = 6.626 \times 10^{-34} \ J \cdot s, \ c = 3 \times 10^8 \ m/s]$

Explain: Photoelectric effect.

The radius of a hydrogen atom in its ground state is $0.53 \ \mathring{A}$. The radius of the $Li^{2+}$ ion in the same state (atomic number = $3$) is ............. $\mathring{A}$.

The electronic transitions from $n=2$ to $n=1$ will produce the shortest wavelength in which of the following species? (where $n$ is the principal quantum state)

The shortest wavelength of $He^{+}$ ion in Balmer series is $x$. Then,the longest wavelength in the Paschen series of $Li^{2+}$ is: