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
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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