The element used by Rutherford in his famous scattering experiment was

When an electron transitions from the $6^{th}$ orbit to the $2^{nd}$ orbit in a $H$ atom,what is the total number of spectral lines (excluding the Balmer series) produced?

The radii of the first,second,third,and fourth Bohr orbits of hydrogen are respectively:

Which of the following is correct regarding Bohr's theory?

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

What is the shortest wavelength in the $Pfund$ series of $He^{+}$ ion?