A glass sheet $\backslash (1\mathrm{.}40\backslash$mu $\backslash$mathrm$\{$~m$\}\backslash )$ thick is suspended in air. In reflected light, there are gaps in the visible spectrum at $556$ nm and $625\mathrm{.}00$ nm $.$ Calculate the minimum value of the index of refraction $\backslash ($ n $\backslash )$ of the glass sheet that produces this effect.

$\backslash ($ n$=\backslash )$