Reststrahlen effect
The Reststrahlen effect (German: “residual rays”) is a reflectance phenomenon in which electromagnetic radiation within a narrow energy band cannot propagate within a given medium due to a change in refractive index concurrent with the specific absorbance band of the medium in question; this narrow energy band is termed the Reststrahlen band. As a result of this inability to propagate, normally incident Reststrahlen-band radiation experiences strong reflection or total reflection from that medium.