light turns on when window ac unit is on
Call a trained electrician and get them to take a look at it.

Electricity is dangerous, and it sounds like you've got mains voltage leaking between at least two circuits - and it's entirely possible that things like the wall or floor of that room are currently electrified, depending on how the leakage is occurring. Trying to fix it without formal training could potentially get you killed or your house burned down - and is potentially illegal, depending on where you are located. I can say with a certainty that it's definitely legal in Australia, at the very least.