Main Question or Discussion Point
what is effective mass? and how does it effect both the electrical and optical properties of a crystaline structure???
Intuitively, effective mass is a concept that uncouples one many body problem into a set of one body problems that are more easy to solve (i mean, of which the Schrödinger equation, ie the SE, is easier to solve).
In vacuum you have Newton's second law relating force, velocity (through acceleration) and mass.
In band structure calculations the effective mass is basically the curvature of the energy-band near the bottom.