Negative refraction

The picture on the left shows how a spoon in a glass of water appears to break at the air-water interface, and then continues inside the liquid slightly shifted to one side, but still keeping the same orientation as in air — left-to-right in this case. This optical illusion is due to the fact that the refractive index of water is different from the one of air — still positive though, as in any other natural material. On the right, there is a basic illustration of what would happen by filling the glass with a liquid with a negative index of refraction: the orientation of the spoon inside the liquid would appear to be diametrically opposite to the spoon in the air, namely right-to-left instead of left-to-right.
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