Yeah, I ended up writing the whole physical background of tir. Basically, it's when the angle of light or los in bryce's case exiting the refractive material is shallow enough so that the resultant refraction (n) of the material causes the final angle of the light/los to be less than the 'escape angle' from the water. Thus it reflects back down like in the image above. Jewlers use this law of nature when calculating the suitable angles for diamons as to throw the light that comes in from all angles out of the top crown thus producing a sparkling/lit effect.