When light travels from one medium to another, the angle of incidence of light in the first medium is called the angle of incidence. There is a certain limit to the value of critical angle till which it allows refraction to happen. For example, for the glass-air boundary, the critical angle is 42º. That means refraction phenomenon happens only when the incident light falls at an angle of incidence within 42º.

If the angle of incidence increases beyond this, it is found that there is no refraction and all the incident light gets reflected at the boundary of the media. This phenomenon is called total internal reflection.