Wave propagation in composite media has attracted a great deal of attention because of its wide applications in science and engineering. In the low-frequency limit, when the wavelength is much larger than the structures in a composite, the wave is unable to resolve the fine structures of the composite and the wave behaves as if it is propagating in a homogenized medium. Thus, an effective medium is usually used to describe composite properties. In recent years, with the development of metamaterials, continuous efforts have been devoted to finding proper effective medium theories in order to theoretically describe various novel phenomena such as negative refraction. Details of these efforts may be found in a recent review and the references therein.
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