Both transmission and reflection can happen at the boundary between two media: in fact both may happen simultaneously - the yellow and cyan rays below. For angles greater than a certain angle, called the critical angle, there is no refraction and all the light is reflected – the grey ray below. This phenomenon is called 'total internal reflection', and only happens when the light is travelling in the denser medium.
If the angle of incidence equals a 'critical angle' labelled
there is no reflection but part of the incident light traces a 'critical path' along the interface between the two media – the blue rays above. This is called the 'critical ray'. The critical angle depends on the material.
The critical angle can be worked out from Snell's law,
For the critical ray
If media 2 is air then
and if media 1 is glass of refractive index 1.4, then
