alternator works fine and light is on what should i do ?

Have the car scanned to see if the computer is recognizing the charging. Some alternators have an internal signal that sends the information. If it is not working, either live with it or replace the alternator.

the professor is right,get it scanned and don't assume a new part is good,especially if it's from a non genuine parts store.

IF it was ok before the belt was changed, have the work rechecked! Possible damaged wiring or other componet.