So dig this animation:
My friends usually balk at the claim that A and B have the same
light value. (But we tried it out in real life too -- see?)
I adapted this illustration from Edward H. Adelson's
original, found
here. Here's his explanation for how it works:
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 measuring the light
coming from a surface (the luminance) is not enough: a cast shadow
will dim a surface, so that a white surface 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.
The first trick is based on local contrast. In shadow
or not, a check that is lighter than its neighboring checks is probably
lighter than average, and vice versa. In the figure, the light check
in shadow is surrounded by darker checks. Thus, even though the
check is physically dark, it is light when compared to its neighbors.
The dark checks outside the shadow, conversely, are surrounded by
lighter checks, so they look dark by comparison.
A second trick is based on the fact that shadows
often have soft edges, while paint boundaries (like the checks)
often have sharp edges. The visual system tends to ignore gradual
changes in light level, so that it can determine the color of the
surfaces without being misled by shadows. In this figure, the shadow
looks like a shadow, both because it is fuzzy and because the shadow
casting object is visible.
The "paintness" of the checks is aided
by the form of the "X-junctions" formed by 4 abutting
checks. This type of junction is usually a signal that all the edges
should be interpreted as changes in surface color rather than in
terms of shadows or lighting.
As with many so-called illusions, this effect really demonstrates
the success rather than the failure of the visual system. The visual
system is not very good at being a physical light meter, but that
is not its purpose. The important task is to break the image information
down into meaningful components, and thereby perceive the nature
of the objects in view.
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