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Application of an electromagnetic wave to a material induces an internal response that is modeled using the material parameters permeability (μ) and permittivity (ε).
This model works because the atoms are much much smaller than a wavelength, allowing us to approximate their response homogeneously.
This means that we aren’t restricted to atoms (and their fixed range of material properties). We can use manufactured periodic structures to achieve exotic (very high, near zero, or even negative) values of permeability and permittivity, as long as the size of the unit cell is sufficiently small relative to a wavelength. These manufactured structures are called metamaterials.