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==Scattering and periodicity==
[[Image:1D-Empty-Lattice-Approximation.svg|thumb|400px|Free electron bands in a one dimensional lattice]]
The periodic potential of the lattice in this free electron model must be weak because otherwise the electrons wouldn't be free. The strength of the scattering mainly depends on the [[geometric topology]] of the system. Topologically defined parameters, like [[Scattering cross-section|scattering]] [[Cross section (physics)|cross sections]], depend on the magnitude of the potential and the size of the [[potential well]]. One thing is clear for currently known 1, 2 and 3-dimensional spaces: '''potential wells do always scatter waves''' no matter how small their potentials are, what their signs are or how limited their sizes are. For a particle in a one-dimensional lattice, like the [[Kronig-Penney model]], it is easy to substitute the values for the potential and the size of the potential well.<ref name=Kittel>
{{cite book |author=C. Kittel |title=Introduction to Solid State Physics |year= 1953-1976 |publisher=Wiley & Sons |isbn=0-471-49024-5 }}
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