When light travels from one medium to another a small amount of the light is reflected, and most is refracted. If light strikes a boundary when travelling from a more dense to a less dense medium
The angle of refraction depends on the angle of incidence. 
At the critical angle C, the angle of refraction becomes 90° and the refracted ray travels along the boundary. For light travelling from a material of refractive index n into air, sin C = 1/n. 
When the angle is greater than the critical angle, you get total internal reflection.
The angle of refraction depends on the angle of incidence. At the critical angle C, the angle of refraction becomes 90° and the refracted ray travels along the boundary. For light travelling from a material of refractive index n into air, sin C = 1/n. When the angle is greater than the critical angle, you get total internal reflection.