The energy required to excite an electron from the first orbit to the third orbit in a hydrogen atom is .......... $eV$.

$A$: An electron revolving around the nucleus does not fall into the nucleus.
$R$: An electron revolving around the nucleus is an orbital electron.

Four lowest energy levels of $H$-atom are shown in the figure. The number of emission lines could be obtained by transitions from these levels. The total number of possible emission lines is:

The ratio of the radius of the second orbit of hydrogen to the third orbit of $Li^{+2}$ is .... ($H$ atomic number = $1, Li$ atomic number = $3$)

Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nanosecond range. If the radiation source has the duration of $2 \, ns$ and the number of photons emitted during the pulse source is $2.5 \times 10^{15}$,calculate the energy of the source.

In a hydrogen atom,electrons are excited to the $5^{th}$ quantum level. How many different spectral lines may appear in the emission spectrum?