The Checker Shadow Illusion

The checker shadow illusion is my favorite illusion, because even though when I know exactly what's going on it still appears otherwise. The checker shadow illusion is an image of a checkerboard of grey and dark grey squares and in one corner there is a cylinder casting a shadow across the board. Inside that shadow the checker board continues its pattern, and so the lightest color square inside the shadow area appears to look like the same color as the lightest shade outside of the shadow area. Yet somehow that light colored square in the shaded area is actually the same color as the darkest colored square on the outside of the shaded area.
One of two tricks that makes us perceive the squares local contrast. The squares surrounding the square in the shadow are darker so the center square appears more similar to the lighter squares outside of the shadow.
The second trick is because of how we see shadows. Shadows have soft edges and so we ignore the gradual darkening of objects while the paint edges have sharp edges that are more distinct.

-Melinda Campanella