Re: Why can�t light be in two mediums at once?

The question as asked is very hard to answer because it in not clear what 
you actually want to know. If the question is about light passing through 
multiple media that are different, then the answer is that yes, light can 
and does travel through more that one medium. Take for example seeing 
light from outside through contact lenses. The contact lens is in direct 
contact with the eye. This represents two different optical media yet the 
light easily travels through the two so people wearing contact lenses can 
see.  This is at 90 degrees incidence or some other very large angle 
relative to the surface of the lens. It becomes more complicated as the 
incidence angle becomes smaller and smaller. This is the critical angle 
scenario you talk about in your question. At this angle something very 
interesting happens that depends of the refractive index of each medium 
and the energy of the light. In order to understand what happens I would 
suggest researching the physics of critical angle reflection which you can 
find at the following web site. This is a geometric physical view of 
reflection which describes what happens but not why it happens. A more 
thorough (and complicated) description is the subject of a topic in 
advanced physics called quantum electrodynamics which I encourage you to 
research if you are interested in a more detailed explanation of 
reflection and transmission of light through multiple media.
 http://www.physicsclassroom.com/class/refrn/u14l3c.cfm