Evanescent wave

An important side effect of total internal reflection is the propagation of an evanescent wave across the boundary surface. Essentially, even though the entire incident wave is reflected back into the originating medium, there is some penetration into the second medium at the boundary. This wave can lead to a phenomenon known as frustrated total internal reflection. Additionally, the evanescent wave appears to travel along the boundary between the two materials, leading to the Goos-Hänchen shift.