$\text{When reflected light is totally polarizrd then reflected and refracred light is perpendicular to each other}$

$\Rightarrow i+r= 90^0 , r = 90^0 -i$

$\text{ From Snell's law } \frac{sin i}{sin r} = n, \Rightarrow \frac{sin }{sin(90^0 -i)} = n$

$\Rightarrow tan i = n , \Rightarrow i = tan^{-1} n$