The wavelength of a spectral line for an electronic transition is inversely proportional to :

The wavelength of a spectral line for an electronic transition is inversely proportional to the difference in the energy levels involved in the transition.

$$\begin{gathered} \mathrm{\Delta E}={\mathrm{E}}_2-{\mathrm{E}}_1;\frac{hc}{\lambda } \\ ={\mathrm{E}}_2-{\mathrm{E}}_1\text{ or }\lambda \propto \frac{1}{\left({\mathrm{E}}_2-{\mathrm{E}}_1\right)} \end{gathered}$$