# Homework Help: Unable to interpret E-k diagrams (still)

1. Jul 19, 2008

### _Andreas

1. The problem statement, all variables and given/known data

Look at the first figure http://www.ee.ui.ac.id/~astha/courses/ts/teksem/effmass.htm" [Broken], 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!

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