Total internal reflection occurs when;

Hint: Reflection is that the change in direction of a wave front at an interface between two different media in order that the wave front returns into the medium from which it originated. Common examples include the reflection of sunshine, sound and water waves.

Complete step by step solution:
When a light-weight travelling during a denser medium is incident at the surface of a rarer medium such that the angle of incidence is bigger than the angle of incidence for the pair of mediums, the ray is completely reflected back to the denser medium. This phenomenon is named the entire internal reflection.
A ray of sunshine passed from a medium of water to air. Light rays are going to be refracted at the junction separating the 2 media. Since it passes from a medium of a better index of refraction thereto having a lower index of refraction, the refracted light ray bends far away from the traditional.
At a selected angle of incidence, the incident ray of sunshine is refracted in such a way that it passes along the surface of the water. This particular angle of incidence is named the angle of incidence. Here the angle of refraction is 90 degrees. When the angle of incidence is bigger than the angle of incidence, the incident ray is reflected back to the medium. We call this phenomenon total internal reflection.

Note:
When the incident ray falls on every face of the diamond such that the angle formed, the ray is bigger than the angle of incidence. The critical value of the diamond is 23°. This condition is liable for the entire internal reflection during a diamond which makes it shine.