God intends to bring about a permanent physical separation of light from darkness. Light weight and light bearing are synonyms, like dark and heavy are. In general heavier, denser materials are darker.
Physical Separation
To discern: בִּין – bin
The Hebrew words bayin - בֵּין and bin - בִּין are very nearly the same word. They're part of the meaning of a root which means: to understand nuance, to separate between.
Matty's Paradigm 