Why do we light a lamp?

Hi,

This is very useful information. Btw as discussed it is said to lit lamp using ghee for 40 days wishing for something to happen, but then since it is not possible for ladies to do it at a stretch is there any harm in continuing after the days in a month to complete the 40 days. Or is it ok if somebody else like my husband can light the lamp if i cannot do it on any day? Pls reply...

Sharmila.