To us it looks like an object with a dark grey top and a white bottom, and our brains insist on seeing the bottom as lighter than the top.
It is a dark grey top with a white bottom. White appears grey when less light hits it. That’s what shadows are.
Uh. No. It’s a gray top with a same shade of gray bottom. There is no shadow (unless half your monitor is under a tree.)
It’s all pixels.
And it is indeed an optical illusion.
