Original sentence: A lense can be polished, the ratio of impurities can be lessened, the shape of the material can be altered, but the control is ultimately limited to the effieciency of the interface.If both permittivity and permeability are negative, the resulting refractive index is negative as well.In dielectric media, which includes air, light bends naturally when the refractive index, n,  is spacially varying.Mathematically, the square of the index of refraction, n2 =  ɛ µ , is given by the product of the electric permittivity ɛ  and the magnetic permeability µ of the medium.When the material can control the intended direction of electromagnetic waves, at will,  then it is possible to bend rays around a hole, and recombine them in precisely the same direction as they entered the medium
Edited sentence: A lense can be polished, the ratio of impurities can be lessened, the shape of the material can be altered, but the control is ultimately limited to the efficiency of the interface.If both permittivity and permeability are negative, the  resulting  refractive index is negative as well.In dielectric media, which includes air,  light bends naturally when the  refractive index, n,  is spacially varying.Mathematically, the  square of the index of refraction, n2 =  ɛ µ , is given by the product of the electric permittivity ɛ  and the magnetic permeability µ of the medium.When the material can control the intended direction of electromagnetic waves, at will,  then it is possible to bend rays around a hole, and recombine them in precisely the same direction as they entered the optical transmission medium