According to the relation

$\mathrm{\Delta }E\propto \frac{1}{\lambda }\propto \upsilon \propto \left[\frac{1}{n_L^2}-\frac{1}{n_u^2}\right]$

$n_{\mathit{L}}\mathit{=}$ lower energy level

$n_u=$ higher energy level

In hydrogen atom the emission of the least energetic photon take place only when change in energy is minimum. This is possible for an electron that is present in $n=6$ and jumps to $n=5$ .