No front can refract in the negative fashion (PIM-NIM).

This shows why no front can bend in a negative way and keep going forward. To do this the front would have to pivot around the "bottom" point - the bottom point would have to stop while all others would keep going at different speeds. But the speed of each point is fixed in each mediaum. So this is not possible. Also, if we considered only half the beam, the "pivot" point would change. But what decides which is pivot point? Nothing. Or maybe the front appears on right before arriving at surface - but that violates causality. So you see, there is NO way one can do this.
Note that this works for any material. Negative signal refraction is just not possible. Period.

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