${\mathrm{\mu}}_{\mathrm{g}}\times \mathrm{sin}{\mathrm{i}}_{2}={\mathrm{\mu}}_{\mathrm{a}}\mathrm{sin}{\mathrm{r}}_{2}\phantom{\rule{0ex}{0ex}}\frac{{\mathrm{\mu}}_{\mathrm{a}}}{{\mathrm{\mu}}_{\mathrm{g}}}=\frac{\mathrm{sin}{\mathrm{i}}_{2}}{\mathrm{sin}{\mathrm{r}}_{2}}={\mathrm{g\mu}}_{\mathrm{a}}.....\left(2\right)$

According to the

Imagine a plane mirror P held normal to MN so that on reflection from mirror, path MN is reversed. The ray would retrace its entire path. For the reversed ray, application of Snell's law at M gives

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