I've been working on problems trying to find the dispersion relation of monatomic and diatomic chains. I'm able to find the dispersion relation, but I don't really understand what it is. I'm pretty sure that it's the relation of the frequency to the wave vector. I'm working through Kittel's Intro to Solid States, and it states that the wave vector is simply 2Pi/wavelength. I'm wondering where the wavelength comes from though. Is that the wavelength of the wave propagating through the crystal lattice? Could someone please explain?