Frustrated Total Internal Reflection (FTIR)

The video below shows an example of frustrated total internal reflection (FTIR).

The idea behind is that the wave in the lower index medium is exponentially decaying.

However, as you can see in the figure above, we can have a scenario where total internal reflection is happening and an evanescent wave propagates along the interface. If we then bring another piece of glass close enough to the first, within the range where the exponentially decaying wave has not dropped to zero amplitude, we can actually get the exponentially decaying wave to couple into the second piece of glass and become a propagating wave again. This phenomenon is known as Frustrated total internal reflection.