# Where is the point of Incidence? As you may know that critical angle is the angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected. So at critical angle, the angle of refraction is 90°. As per the fundamentals of Ray optics, if we incidence a ray in reverse ie. from air to glass at an angle of 90°, the angle of refraction should be C ( critical angle ). But where exactly is the point of incidence if the angle of incidence is 90°? (You can try this with a glass cube and a laser. When I tried, the light is seemed to glare throughout the interface but did not observe any refraction)

In the context of the above, the concept of an incident beam precisely at $$90^\circ$$ is already an ill-defined concept, before you have ever inquired about where it arrives at the interface. Instead you will have to take a limit as the angle tends to $$90^\circ$$, and if you also require that no part of the incident beam is at more than $$90^\circ$$ to the normal, then you find you are talking about a beam whose width tends to infinity as the angle tends to $$90^\circ$$, because simultaneously the direction has to become more and more precise. Consequently the width of the region on which the incident light meets the interface also tends to infinity.