Adelson’s “Checker-shadow illusion”
What to do
You will, in all likelihood, perceive the above picture as a 3-dimensional scene, where a greenish cylinder stands on a checkered plane; light comes from top left.
Now try the first button (“Question 1”) on the top right. Two patches will be indicated, and you are asked to compare their relative brightness. I think we can all agree that the top one is darker.
A second later a yellow mask will appear. Now you can compare the two patches without the 3-dimensional context. Now the brightness will be identical. The luminance is identical at all times, but the brightness (=apparent luminance) differs, depending on context.
When interpreted as a 3-dimensional scene, our visual system immediately estimates a lighting vector and uses this to judge the property of the material.
Adelson EH (1993) Perceptual organization and the judgment of brightness. Science 262:2042–2044
Adelson EH (2000) Lightness Perception and Lightness Illusions. In The New Cognitive Neurosciences, 2nd ed., M. Gazzaniga, ed. Cambridge, MA: MIT Press, pp. 339–351