Another example is mechanical waves in the solar photosphere : The waves are damped ( by radiative heat flow from the peaks to the troughs ), and related to that, the energy velocity is often substantially lower than the waves'group velocity.
2.
Russian mathematician Victor Veselago predicted that a material with simultaneously negative electric and magnetic polarization responses would yield a negative refractive index ( an isotropic refractive index of " 1 ), a left-handed medium in which light propagates with opposite phase and energy velocities.
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