Why can't waves (light) be in two mediums at the same time?

[i_m_mimi]

Technically the critical angle is impossible to occur, instead internal reflection occurs. This is because light can't be in two mediums at the same time. WHY?
What happens when the light particle/wave is in between two other medium particles?

Any links to help me answer this question would be helpful.

 

[Andy Resnick]

Could you be referring to surface plasmons?

http://en.wikipedia.org/wiki/Surface_plasmon_resonance

 

[i_m_mimi]

Could you be referring to surface plasmons?

http://en.wikipedia.org/wiki/Surface_plasmon_resonance

Unfortunately, I have no idea what it is talking about. Too many big words. I'm only in the 10th grade.
I don't really understand it, but I think you're on to something.

While incident light is totally reflected the electromagnetic field component penetrates a short (tens of nanometers) distance into a medium of a lower refractive index creating an exponentially detenuating evanescent wave.

This seems like something that might help me answer the question, but I don't know what it means.