Shinwari,Mariam,Ivan text:
Total Internal Reflection
is a phenomena which occurs when light propagates from the medium with greater index of refraction in the medium with lower index of refraction. From Snell's law we know that the light in this case refracts away from the normal. That means that there is an angle of incidence for incoming ray where all the light will not leave the medium with higher index of refraction, but instead it will reflect in direction perpendicular to the normal (i.e. in the plane of the boundary). If the angle of incidence is larger than this critical angle, all the light is reflected back into the medium with higher index of refraction. We can obtain value for critical angle mathematically using following equation: 
Where n2 is index of refraction of the medium with higher index.
Image of multiple total internal reflections courtesy of Wikipedia. 
Practical Use of Total Internal Reflection