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In this module we think about how the frequency of lattice vibrations in solids varies with wave vector by making cartoons of how the atoms displace at different wave vectors. (DISCLAIMER: This is not meant to replace the mathematical derivation that you can find in any standard text book, but hopefully might help you with some extra intuition.) We start with a diatomic molecule, and look at the classical and quantum mechanical solutions for its vibrational energy. We then extend the discussion to a periodic solid, again for the classical and quantum mechanical cases.