When light travels from one medium to other, it gets refracted. The relation between the angle of incidence and the angle of refraction is given by the Snell's law. The Snell's law is described in the following figure.
If the angle of incidence, is greater than the critical angle, given by
where
and
are the refractive indices of the two media as shown in Fig., then the light is Total Internally reflected in medium 1. There is no refracted ray in that case.
is greater than the critical angle, 
given by 
and
are the refractive indices of the two media as shown in Fig., then the light is Total Internally reflected in medium 1. There is no refracted ray in that case. 
