Question

In analyzing the motion of an elastic wave through a material medium,we often ignore the molecular structur of matter.when is this justified and when is it not?
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Answer 1

Everything is dependent on the ratio between wavelength L and crystal lattice constant a: if L much greater than distance between neighboring atoms of medium, we can ignore molecular structure of matter. But if wavelength is small (wave vector k is large) then we need to consider discrete structure of crystal lattice.