A 12 . 5   eV electron beam is used to bombard gaseous hydrogen at room temperature. The number of…

According to Bohr's postulates, if an electron absorbs a photon with energy equal to the difference between the energies of an excited state and the ground state, it will jump to a higher energy orbital.

Considering that the energy of the colliding electron is $10.2 \mathrm{eV}$ or $12.09 \mathrm{eV}$, depending on how much energy is lost during the collision. $n=3$ is the maximum excited state. Hence, there are spectral lines in a total of

$\begin{array}{rcl}{}^{3}C_2& =& \frac{3!}{2!1!}\\ & =& 3\end{array}$