Well have is power? P=IV. As the above poster said, you need to assume it is directly proportional to the brightness, and the above equation helps make sense of that. If the current through the load increases, what exactly is happening? More charge is passing through each second right? How is that going to effect the brightness?
This sounds like a homework question really. You'll learn nothing by us giving you the answer. Best to research it yourself now that you've been given a starting point.