Where is Total internal reflection occurs when a beam of light Answer?

Total internal reflection occurs under two conditions;

1)the light must be traveling from more dense to less dense mediums

2)the light ray must hit the surface at an angle ,called the critical angle, or larger, as measured from a perpendicular (surface normal) line to the surfaces. The critical angle is typically in the 40 - 50 deg range, but specifically depends on the densities of the two media.