Why does the illusion work?
The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just http://www.blogger.com/img/blank.gifmeasuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surfacehttp://www.blogger.com/img/blank.gif in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray "paint" that belongs to the surface.
data source: mit.edu