You notice that when light is travelling from a dense medium (higher refractive index) to a less dense medium (lower refractive index), the angle of refraction is always bigger than the angle of incidence. In this situation it is possible to have an angle of refraction of 90°, where the light does not enter the second medium, but travels along the boundary.
If the angle of incidence is increased further, all of the light will be reflected back into the first medium. This is called total internal reflection. The slides introduce and outline total internal reflection, the critical angle, and it’s application in fibre optics. Instructions for an investigation are also included.