SAT II Subject Test Physics: Geometrical Optics - Critical angle and Total reflection

Total reflection can only occur when the light is traveling from denser medium to rarer medium. From water to the air for example. A good excuse to buy a water proof torch that you can experiment in the swimming pool. :)

When light travels between two different medium, both reflection and refraction occur normally. Since the light is traveling from denser medium to the rarer medium, the angle of refraction will be greater than the angle of incident. We can however found an angle of incident that the angle of refraction becomes 90 degree, i.e parallel to the surface. In this case, no refraction occurred, all lights are reflected. This is called total reflection and the incident angle at that point is called critical angle.

In summary, when the incident angle is 0, not reflection occurs, it is total refraction. However when the incident angle reach the critical angle, no refraction occurs, it is total reflection. When the incident angle is between 0 and critical angle, the greater the angle of incidence, the greater the amount of reflection.