Subject: Why can’t light be in two mediums at once?

Since the critical angle is the incident angle that will give total internal reflection. 
This ’90 degrees’ that occurs when light is at the critical angle never occurs, but it 
is automatically reflected. My teacher told me this is because light can’t be in two 
mediums at once, and ‘90 degrees’ at a media boundary is being at two mediums 
at once. What exactly is happening when light tries to be in two mediums?

Re: Why can’t light be in two mediums at once?