FWIW, slide 21 from this presentation on the site you linked has the picture I have in mind since at that size scale the glass is effectively flat. http://ab-i

FWIW, slide 21 from this presentation on the site you linked has the picture I have in mind since at that size scale the glass is effectively flat. http://ab-initio.mit.edu/photons/tutorial/L1-bloch.pdf http://ab-initio.mit.edu/photons/tutorial/L1-bloch.pdf Feel free to correct if this is oversimple, I am vaguely familiar with this stuff but it was by no means a speciality.