  Experimental evidence of wave properties of dendritic flux avalanches in
superconducting films is reported. Using magneto-optical imaging the
propagation of dendrites across boundaries between a bare NbN film and areas
coated by a Cu-layer was visualized, and it was found that the propagation is
refracted in full quantitative agreement with Snell's law. For the studied film
of 170 nm thickness and a 0.9 mkm thick metal layer, the refractive index was
close to n=1.4. The origin of the refraction is believed to be caused by the
dendrites propagating as an electromagnetic shock wave, similar to damped modes
considered previously for normal metals. The analogy is justified by the large
dissipation during the avalanches raising the local temperature significantly.
Additional time-resolved measurements of voltage pulses generated by segments
of the dendrites traversing an electrode confirm the consistency of the adapted
physical picture.
