The angular momentum of an electron in the $7^{th}$ Bohr orbit of a hydrogen atom is given by:

The four lowest energy levels of the $H$ atom are shown in the figure. The number of emission lines will be ...... .

What is the ratio of time periods $(T_1 / T_2)$ in the second orbit of a hydrogen atom to the third orbit of a $He^{+}$ ion?

State whether the following statements are true or false:
$(i)$ The $Bohr$ model cannot explain how atoms form molecules.
$(ii)$ The $Bohr$ model is accepted as the planetary model.
$(iii)$ Electrons in an atom do not absorb or emit energy while moving in a stationary orbit.
$(iv)$ The spectral lines of the $Brackett$ series are found in the ultraviolet region.

The longest wavelength line in the Lyman series of the $H$ atom spectrum is $:-$

The work function of a metal $M$ is $6.3 \ eV$. The wavelength of the incident radiation required to just eject the electrons from its surface (in $nm$) is