...
This give stuff their characteristic properties. Such as conduction, insulation, and semi conductor, and optical properties. The electrons closer to the bottom of the graph are deep in the atomic well and need tons of energy to wiggle or kick out. The ones at the top of the conduction band are easier to kick out and or wiggle with incoming light.
The energy band gap can move and change due to temperature. So we can manipulate the temp so we can take snap shots of the phase transistion.
