1. The problem statement, all variables and given/known data Look at the first figure here, and pretend that the conduction bands on the left and right don't exist. As far as I know, the energy of a free electron is directly proportional to the square of its wavenumber. Yet, it's possible for there to take place a direct absorption process between the highest point of the valence band and the lowest point of the conduction band (the band right above the valence band). Here's what I have struggled with for hours to understand: If the electron after the absorption still has a wavenumber that equals zero (the diagram indicates that the lowest point of the conduction band is where k=0), how can its energy not still be zero? I'm sure there's some fundamental misunderstanding on my part here, but I really can't see what it is. Help!