Critical angle

The smallest angle of incidence at which total internal reflection occurs; that is, at which light passing through a material of a higher refractive index will be reflected off the boundary with a material of a lower refractive index. At lower angles, the light is refracted through the cladding and lost. Due to the fact that the angle of reflection equals the angle of incidence, total internal reflection assures that the wave will be propagated down the length of the fiber. The angle is measured from a line perpendicular to the boundary between the two materials, known as normal.