a1) c'e` una luce
is a contraction of:
a2) ci e` una luce
The particle /ci/ in this case has the meaning of one of: qui, la` (here, there)
The word /cio`/ which has the meaning that you said is never changed.
There is another word: cioe`, that derives from cio` and means /it means/, somebody uses
cioe` as a meaningless filler in the conversation, which (in my view) is quite ugly to listen to.
/ci/ has also other meanings, like: /to us/.