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The interface between two facets of an artificial material known as a “Weyl phononic crystal” can not only negatively refract an airborne sound wave, it does so without reflecting it at all. This hitherto unseen wave behaviour could be important for fundamental studies of condensed matter and find many practical applications in acoustics, electronics and optics too.

Refraction occurs when waves pass from one medium to another and change direction. Although part of the wave’s energy passes through the interface between the two different media during this process, the rest of it is inevitably reflected from the interface. Refraction and reflection are two fundamental interfacial wave phenomena and are exploited when making devices like optical lenses.

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