Brightness on MBP changes randomly when watching videos full screen

Im sure there is a simple solution to this but I'm new to Mac. I got my MBP about 2 weeks ago and i notice this when I watch videos or tv on full screen (whether its Youtube, Netflix or a DVD). The brightness would dim and then suddenly theres a burst of brightness. It doesn't white out the screen or anything but its really annoying especially if I'm in a dim room, also the Apple logo light on the back getting brighter and darker is really obvious. Any ideas on how to sort it?