In a mixture of $\mathrm{H}-\mathrm{He}^{+}$gas ( $\mathrm{He}^{+}$is singly ionized He atom), H atoms and $\mathrm{He}^{+}$ions are excited to their respective first excited states. Subsequently, H atoms transfer their total excitation energy to $\mathrm{He}^{+}$ions (by collisions). Assume that the Bohr model of atom is exactly valid.
The wavelength of light emitted in the visible region by $\mathrm{He}^{+}$ions after collisions with H atoms is