as per diagrams in textbooks, if incidence angle = critical angle then the ray should travel straight until it reaches the surface of the denser medium and then go tangentially to the boundary something like the following http://www.antonine-education.co.uk/Physics%20A%20level/Unit_2/Waves/Refraction/Refract_8.gif [Broken] if i now place a mirror vertically against this ray (which is tangentially traveling) , will it exactly trace its path? As per 'principle of reversibility' it should. However, it cannot 'know' where exactly it had bent. therefore it seems to me that the behavior of ray with i=90' is somewhat undefined ( like tan 90'). please tell me experts where i am unclear.