Suppose that all the vibrational branches of a one-dimensional crystalconsisting of two different masses with equal spacing, a=3.0 can berepresented by a single optical mode branch and a single acoustical modebranch. If the frequency of the long-wavelength optical waves is 2x1013 rad/sand the maximum frequency of acoustical waves is 1013 rad/s, what is thegroup velocity of long wavelength acoustical waves?

For Long wavelengths λ is much greater than 2a, where a is the distance between two masses frequency…

Answered: Suppose that all the vibrational…

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