Indirect absorption processes -- need help! On page 188-190 in Introduction to Solid State Physics by C. Kittel, I read I don't get it. I thought it was as simple as this: the highest point in the valence band and the lowest point in the conduction band have two different energies, and they therefore have two different wavevectors (this is because different energies gives different wavenumbers). The energy difference is E, and therefore the least energetic photon possible of exciting an electron from the valence band to the conduction band must have this energy. But according to the above quote, the energy of the photon is enough, but not the magnitude of the wavevector. This leads me to think that the photon wavenumber is different from a matter wave wavenumber, which is 2pi/lambda. Is this correct?