Refraction has a symmetry; on going from glass to air light at the glass air surfaces refracts away from the normal. If you turn the light source around and make the former refracted beam the incident beam then its refracted angle will be the former incident angle. So at critical angle, do the same trick, skim the light along the surface. how does it know what to do? carry on skimming or refract into the glass at 41.8 degrees.
what then troubled me even more is that if a beam skimming the surface can spontaneously refract into the glass the why doesn't light refracted at 90 degrees (due to i being critical) refracted back in again .
Does this mean that the whole idea of a critical angle is a limiting idea ... if below critical light refracts out the block, if incident angle is above the critical we get TIR - so what actually happens at the boundary between these two?