If your MacBook has a dim or slightly less brighter display please check that you have not obscured the FaceTime(webcam) on the top of your MacBook Air's screen. An ambient light sensor is also contained in that black cutout. By blocking that you're essentially telling your MacBook that you're always in a dark room with no light and to dim the display to soften glare and save battery life.


Covering the camera with bare tape only can also damage the lenses and may in some instances with higher strength tapes pull the module out. 

If you must cover the camera please use an opaque or black piece of paper and tape on either side. A small band-aid (regular, non medicated) can also be used. Do not place tape of any kind directly over the camera module. Please refrain from using the plastic commercially made sliding covers. They will sit proud of the screen clamshell and impact the touchpad which will cause it to crack when excess pressure is placed on it.