The ray of light entering the rarer medium from the denser medium
bends away from the normal; i.e. angle of refraction (r) in rarer medium >
angle of incidence (i) in denser medium. Therefore as shown in fig. 10, as i
increases, r increases and since r > i; the ray I2P incident at angle 'C'
is just emerging out into the rarer medium and its path is parallel to the boundary.
This angle 'C' is called the critical angle of the given denser medium in relation
to the given rarer medium. The critical angle can be defined as the maximum
angle of incidence possible in denser medium for the ray to just emerge out
into rarer medium.