Why does my my door have odd circular lighting effect on it? All of my other meshes have normal lighting on then but when it comes to my shower door it creates this odd circular effect that I just can't seem to remove. 



The door is just a cube, however when I duplicate the single front face of the cube as shown here the lighting looks normal. When I extend the face to make a cube it adds that circular effect. 



I haven't applied any materials to any of these objects yet so I'm not quite sure what is going on here.
I'm sorry I'm a little new to this. I tried creating a cube with the exact dimensions and it turned out fine. I've discovered that the lighting behaves in this way if you changed the shading here to 'Smooth' as opposed to 'flat'.