When a light Ray travels from a dence medium to a rare medium, the ray gets refracted. At some angle of incidence the refracted ray travels on the surface between the 2 mediums. This angle (in the dence medium) making the refracted ray to travel on the surface is called the critical angle.
The critical angle is produced for the maximum angle the ray can produce in the dence medium without being totally internally reflected.
And for different materials the critical angle takes different values. The critical angle for a diamond in air is 24 degrees
( while the critical angle for glass is 41 degrees.)
Hope this helps.
