make me understand the principle of reversibility of light......pls.....fast need urgently...  
 
     $$\begin{gathered} {\mathrm{\mu }}_{\mathrm{g}}\times \sin {\mathrm{i}}_2 ={\mathrm{\mu }}_{\mathrm{a}} \sin {\mathrm{r}}_2 \\ \frac{{\mathrm{\mu }}_{\mathrm{a}}}{{\mathrm{\mu }}_{\mathrm{g}}}=\frac{\sin {\mathrm{i}}_2}{\sin {\mathrm{r}}_2}={\mathrm{g\mu }}_{\mathrm{a} }.....\left(2\right) \end{gathered}$$


According to the principle of reversibility of light, when final path of a ray of light after any number of refractions and refractions is reversed, the ray retraces its entire path.
     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