In the E-k diagrams for a semiconductor material,what exactly is k? I have read that it is a propagation constant,a wave vector. But does movement in the semiconductor material represent corresponding changes in k? I mean to ask if changes in k represent the physical movement in the semiconductor material(from an observer's point of view). Also does that parabolic shape in E-k diagrams repeat itself with k periodically? Most of the texts I have seen give only one parabola(though another one for the valence band). Is it arbitrarily chosen that the top of the valence band has an energy equal to 0? In Bohr's model we took the energy at an infinite distance from the atom to have an energy equal to 0.