The simple answer: the brain is tricked into thinking the dark object is smaller because of how the brain perceives color. The human mind is a complex array of nerve bundles sending electrical signals from the brain to the body. The brain stores memories, controls our breathing and heartbeat, and lets the body move. It is the most essentially organ the body has. Our brain is the most complex, advanced piece of bio machinery in existence. Constant study is done to try and understand how the brain works. New discoveries are made all the time. Though, more often than not, these discoveries, while providing a few answers, only give way to a seemingly unceasing amount of questions. Ian Stewart once said, “If our brains were simple enough for us to understand them, we’d be so simple that we couldn’t.” Essentially, our brain is a mystery we can only hope to partially understand.
One secret humanity has uncovered, at least in part, is how the brain perceives color. Color is viewed through the eye. The eye relies on rods and cones to be able to decipher color. There are three facets to color that the human eye picks up on: the actual color (ex: red, blue, yellow, etc.), the value, or brightness of the color, and finally, the saturation, or the depth of the colors expressed. Another way to defined color saturation is the intensity of the colors within a picture or photo.
Saturation is the key component when answering the question above. When the brain processes an image with low saturation, it automatically perceives the picture to be smaller than it actually is. Conversely, if an object or photo has high saturation, or color intensity, then the object appears to be slightly bigger than it actually is.
Why is this? Take an example of a pure black block and a pure white block, both exactly the same size, sitting on a table. The white block appears to be slightly larger due to having a higher saturation of color. White is naturally more intense than black. Why does the brain perceive it to be this way? When the eye takes in light, viewing these two blocks, the white light reflected to the eye causes the area covered by the light from the white block to spread a little on the eye. While the block is not actually larger, the eye perceives it to be so due to the ‘excitement’ caused by the intense saturation. This effect is called irradiation.
The logical conclusion when thinking of the black block is the exact opposite. When looking at an object with low saturation, the area affected by that light shrinks. This is technically true, though not because light with low saturation. Rather, the light with heavier saturation around the black block encroaches on the area taken up by the block. This results in the black block appearing smaller than it actually is due to the higher intensity of the light surrounding it.
Other Optical Illusions:
Optical illusions occur all around us. The are fascinating to look at, the juxtaposition from the eye telling you one thing and logic another is really fun.