Look closely at the image we show you and answer the next question.
Which of the two boxes TO ó B is it darker?
Although it seems incredible, Both boxes are exactly the same color. To see it more easily, we have drawn a gray strip that crosses both cells. You will see that there is no color difference between the strip and the cells TO Y B
The explanation for this phenomenon is as follows: The visual system has to determine the color of objects in the world. In this case the problem is to determine the gray color of the boxes on the floor. Only the measurement of the light that comes from a surface (luminance) is not enough to distinguish them since the shadow obscures a surface, so that the lighter surface in the shadow may reflect less light than a full dark surface light. The visual system uses several tricks to determine where the shadows are and how to compensate for their existence in order to determine the gray tone of the "paint" that belongs to the surface.
The first trick is based on local contrast. Whether or not it has shade, a box that is lighter than its neighbors is probably lighter than average and vice versa. In the figure, the box B located in the shadow zone is surrounded by dark squares and therefore, although the box is physically dark, it is lighter than its neighbors. The boxes TO instead, it is outside the shadow and on the contrary, it is surrounded by lighter squares, so it seems darker in comparison.
A second trick is based on the fact that shadows often have smooth edges, while paint boundaries (such as squares) often have sharp edges. The visual system tends to ignore gradual changes in the level of light, so that it can determine the color of surfaces without being fooled by shadows. In this figure, the shadow looks like a shadow, not only because it is diffuse but because the object that projects it is visible.
As with many of the so-called optical illusions, this effect really demonstrates success instead of visual system failure. The visual system is not very good at being a physical light meter, since this is not its purpose. The important task is to break the image information into significant components and therefore perceive the nature of the objects in sight.
This illusion was created by Edward H. Andelson.
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