A point source S is placed at the bottom of different layers as shown in Fig. the refractive index of bottom most layer is ${\mathrm{\mu }}_{\mathrm{o}}$. The refractive index of any other upper layer is

$\mathrm{\mu }\left(\mathrm{n}\right)={\mathrm{\mu }}_{\mathrm{o}}-\frac{{\mathrm{\mu }}_{\mathrm{o}}}{4\mathrm{n}-18}\mathrm{where}\mathrm{n}=1,2,...$ A ray of light starts form the source S as shown. Total internal reflection takes place at the upper surface of a layer having ir equal to