When the angle of refraction is 90 degree at that time the angle of incidenceis called critical Angle.
Total internal reflection happens when all the light rays they get reflected from a surface such as inside a raindrop
When ray of light travels from an optically denser medium into an optically rarer medium, the angle of incidence in the denser medium for which the beam just emerges into the rarer medium(angle of refraction in the rarer medium is 90°) is called the critical angle for the given pair of media.
1 it is neccesary the angle of incident is greater than critical angle
2 the medium is dense
Angle of incidence should be greater than critical angle