Critical Angle

The critical angle is the angle of incidence where the angle of reflection is 90◦.
The light must shine from a dense to a less dense medium.

If the angle of incidence is bigger than this critical angle, the refracted ray will not emerge from the medium, but will be reflected back into the medium. This is called total internal reflection.

Total internal reflection takes place when
• light shines from an optically denser medium to an optically less dense medium.
• the angle of incidence is greater than the critical angle.