Every human being has a soul, a pure entity that seeks to be lit with the remembrance of the creator (Allah). When it receives that Divine light, it lights up, not just the chest of its possessor, but also other people around it. The stronger the combination of the light of inner purity and the outer light source, the greater its reach.
Now think what happens when the glass orb gets dirty. It blocks the light. The dirtier the glass is, the dimmer the lamp gets. If all of the glass orb gets covered with dirt, the niche is left in complete darkness. Although there still is light inside the glass, that light is of no use now.