A 12.5 eV electron beam is used to bombard gaseous hydrogen at room...

$$\begin{aligned}&\mathrm{E}=\frac{\mathrm{hc}}{\lambda} \Rightarrow \lambda=\frac{\mathrm{hc}}{\mathrm{E}}=\frac{6.62 \times 10^{-34} \times 3 \times 10^8}{12.5 \times 1.6 \times 10^{-19}}\\ &=993 \mathrm{~A}^{{}^{\circ}}\\ &\frac{1}{\lambda}=\mathrm{R} \left(\frac{1}{\mathrm{n}_1^2}-\frac{1}{\mathrm{n}_2^2}\right)\\ & \end{aligned}$$(where Rydberg constant , \mathrm{R}=$1.097$\times$10^7$ ) or, \frac{1}{993 \times 10^{-10}}=$1.097$\times$10^7$\left(\frac{1}{1^2}-\frac{1}{\mathrm{n}_2^2}\right)$Solving we get$\mathrm{n}_2=3$Spectral lines Total number of spectral lines$=3$Two lines in Lyman series for$\mathrm{n}_1=1, \mathrm{n}_2=2 and \mathrm{n}_1=1, \mathrm{n}_2=3$and one in Balmer series for$\mathrm{n}_1=2, \mathrm{n}_2=3$