When the angle of incidence is increased, the bending of refracted ray also increases. However, the maximum value of the angle of refraction can be 90°. The angle of incidence in the denser medium for which the angle of refraction in rarer medium equals 90º is called the critical angle, iC. The refracted ray then moves along the boundary separating the two media.
If the angle of incidence is greater than the critical angle, the incident ray is reflected back in the same medium. Such a reflection is called Total Internal Reflection and the incident ray is said to be totally internally reflected. For total internal reflection to take place, the following two conditions must be satisfied:
(sin i)/(sin r) = µ12 (Snell's Law)
(sin iC)/(sin 90°) = µ12
sin iC = µ12
sin iC = 1/µ21