As the angle of incidence increases, the angle of refraction also increases.  Eventually the angle of refraction is big enough so that it is bent into the surface of the boundary.  This means that none of the energy goes into the second medium, all of it is trapped in the first medium.  We call this total internal reflection.  This is really big in the field of fiber optics.

Total internal reflection will occur at the angle where the refracted ray stays in the boundary.  This is called the critical angle.  The critical angle is given by the formula:

The nice thing is you will are given something similar to this equation in your data booklet.  Can you see it?  What assumptions are made when they write it that way?

·        What is the critical angle for water and air?