Laser light is generated by energizing Helium-Neon gas in chamber. The ground and metastable states of Helium are $0\text{eV}$ and $20.61\text{eV}$ respectively. The ground, higher and metastable energies of Neon are $0\text{eV}$, $18.70\text{eV}$ and $20.66\text{eV}$ respectively. The values of speed of light, Planck constant and charge of electron are $3\times {10}^8\text{m/s}$, $6.625\times {10}^{-34}\text{Js}$ and $1.6\times {10}^{-19}C$ respectively.
Wavelength of laser light generated in this process is

The correct option is C $633.8\text{nm}$
The transition for the major element $Ne$ in case of $He-Ne$ laser is to be considered, since $He$ is present just to excite the $Ne$ atoms. And the transition between higher and the metastable state is kept on account as the atom attains stability in the metastable state.
So, Energy difference
$=(20.66-18.70)\text{eV}=1.96\text{eV}$
We know, $E=hv$
$1.96\times 1.6\times {10}^{-19}=\frac{6.62\times {10}^{-34}\times 3\times {10}^8}{\lambda }$
$\lambda =\frac{6.62\times {10}^{-34}\times 3\times {10}^8}{1.96\times 1.6\times {10}^{-19}}$
$=63329\times {10}^{-7}=633.29\times {10}^{-9}$
$\simeq 633.8\text{nm}$