I've been having a very difficult time understanding why band structure is expressed in terms of momentum/crystalline directions. I've included a picture of the band structure of silicon so that you can better understand my question. I think I understand basic crystalline structures and the meaning of crystalline directions, but I don't understand what relevance this has to the band structure. For example, Si is an indirect semiconductor because its maximum for the valence band is in the (000) direction and its minimum for the conduction band is in the (100) direction... so they don't line up. But what does this really mean? When light hits silicon and excites an electron into the conduction band, does the electron begin to travel through the material in a different direction than the direction in which light struck the silicon? I hope my question makes sense to you... Please help me if you can! I'm a chemist taking this type of physics for the first time and I feel lost... Thank you so much for your help!!!