In passing from a slow to a fast medium, total reflection will occur if the angle between the incoming ray and the normal vector of the boundary surface is larger than the critical angle. In that case the incoming angle is equal to the outgoing angle.

The red line indicates the surface boundary which is hit by a ray, and the green line shows its normal vector. The angle between the ray and the surface normal is shown as an orange disk segment. The angle between refracted or reflected rays and the surface normal is shown as a cyan disk segment.