black shadow in the macbook air screen

I have a macbook air early 2015 model. I found that bottom part of the screen is darker than the rest of the screen. There is about 5mm thick dark patch along the bottom edge. It seems that the screen near the bottom edge glows little less than the other portion of the screen. It is visible only when I read a pdf file or browse internet in the full screen mode. It almost disappears when the brightness is the maximum. Is it usual? Please let me know If I need to carry it to the apple store.