Hi everybody. I'm new here and, first of all, sorry for my bad english :-D I'm studying photoelectric absorption in semiconductors. The book (and professor too) says that, in the conservation law: ki + kph = kf (where ki and kf are wave vectors of initial and final electron state, and kph is the wave vector of incoming photon) we can neglect kph because it is ≈ 2π/λ, whereas ki and kf are ≈ 2π/a, and λ>>a. (a is the length of unitary cell in real space). But I think that this assumption is good only if electron is at the edge of Brillouin Zone; if the initial and final electrons are near [itex]\Gamma[/itex]-point, they should have a very little wave vector, comparable with kph, making the approximation not valid. what is wrong in my words?