Optical fiber is made of glass or plastic and transmits signals in the form of light.The speed of light depends upon the density of the medium through which it is traveling.Higher the density, Slower the speed.
If a ray of light traveling through one medium enters into another medium then its speed changes, making the ray to change its direction.This change is called refraction.When a beam of light moves from a denser medium to a low dense medium, as the angle of incidence increases angle of refraction also increases
It is the angle of incidence where the refracted angle is 90 degree.
Total internal Reflection: Fiber optic cable works on this principle.
When a light ray passes from one medium to another the ray is refracted at the boundary of the medium. For angle of incidence above the critical angle the light is refracted back (reflection) to the same medium. None of it escapes to another medium.Thus a light ray incident at or above the critical angle is trapped inside the fiber and can propagate for many kilometers with virtually no loss.