Dec. 10, 2013
It’s hard to comprehend, but even though our eyes tell us that one block is darker and the other is lighter, we shouldn’t believe them.
The two blocks are, in fact, the exact same color and only appear different due to the effect of the darker and lighter shading across the middle.
Place one finger along the center, blocking the darker and lighter parts, and your eyes will begin to tell the truth.
This illusion was documented by experimental psychologist Tom Cornsweet in the 1960s, hence its name: the Cornsweet illusion.
Cornsweet, who is best known for his work in visual perception, noted that humans perceive color and shade of 3D images in a certain way depending on the way the item is lit and the way shadows fall.
Source: http://www.dailymail.co.uk/news/article-2521023/Dont-believe-eyes-These-blocks-SAME-shade-gray.html#ixzz2n2bwQOXG