Total internal reflection can take place only if

According to Snell’s Law, $\frac{\sin i}{\sin r}=\frac{{\mu }_2}{{\mu }_1}$

where r = 90º for particular incidence angle called critical angle. When the incidence angle is equal to or greater than i_c , then total internal reflection occurs. It takes place when ray of light travels from optically denser medium $({\mu }_1>{\mu }_2)$ to optically rarer medium