The energy required for the ionisation of a hydrogen atom is $13.6 \ eV$. What is the energy required for the ionisation of $He^+$ ion (in $eV$)?

The visible range of the hydrogen spectrum contains which of the following series?

If the $I.E.$ of $He^{+}$ is $54.4 \ eV$,then -

The electron in a hydrogen atom is in the first Bohr orbit $(n=1)$. The ratio of transition energies,$E(n=1 \rightarrow n=3)$ to $E(n=1 \rightarrow n=2)$,is

The potential energy of an electron in the hydrogen atom is $-6.8 \ eV$. Indicate in which excited state the electron is present.

The threshold frequency of a metal is $10^{15} \ s^{-1}$. The ratio of maximum kinetic energies of the photoelectrons,when the metal is irradiated with radiations of frequencies $1.5 \times 10^{15} \ s^{-1}$ and $2.0 \times 10^{15} \ s^{-1}$ respectively,is