Why does my screen dim if I cover the camera

I have a macbook air 2014 and I covered the camera for privacy reasons. I have adaptive brightness on, and my screen brightness is normal but it still changes. How can it sense the light outside if my camera is covered???? Is there a hidden camera on my computer? By the way, I have an android phone and I was given this computer by my school. The reason my name is apleisbad because it wouldn't let me put the word apple and bad in my username, so i tricked it.