The Gradient Effect
For the Gradients Optical Illusions, the blocks are the same shade even though they don't seem to be. They have a shading that gradually gets darker. This is called a gradient. The dividing line in this picture forms where the dark half meets the light half. When this border is uncovered, you see 2 very different shades but when it is covered you see the same shade because your brain can't detect a distinct edge and mistakenly assumes that each block is filled with a uniform shade of gray.
DiSpezio, Michael A. Optical Illusion Magic: Visual Tricks & Amusements. New York: Sterling Pub., 1999. Print.
"Gradients." Optics For Kids. N.p., n.d. Web. 16 Mar. 2015.
"Why Are Our Brains Fooled By Optical Illusions?" Science Experiments RSS. N.p., n.d. Web. 17 Mar. 2015.