Your eyes are lying to you
Which square is darker: A or B?
Your brain tries to determine the TRUE color of objects, independent of lighting. It "knows" B is in shadow, so it compensates by perceiving it as lighter than it appears.
Square A is surrounded by lighter squares, making it seem dark. Square B is surrounded by darker squares (in shadow), making it seem light. Relativity is everything!
The soft gradient of the shadow tells your brain "this is a shadow." Your visual system then "subtracts" the shadow to reveal what it thinks is the true surface color.
Your brain interprets the image as a 3D scene with a cylinder casting a shadow. This context changes everything about how you perceive the colors.
The illusion only works when your brain sees it as a 3D scene with lighting. Break the context, break the illusion:
Edward H. Adelson, Professor of Vision Science at MIT, created this illusion in 1995. It became one of the most famous demonstrations of how human vision processes lightness and shadow.
Adelson's work at MIT's Perceptual Science Group focuses on understanding how the brain reconstructs the physical world from ambiguous visual information. This illusion perfectly demonstrates that we don't see "raw" pixel values—we see the brain's best guess about reality.
You don't see the world as it IS—you see your brain's interpretation of it.
This isn't a bug; it's a feature! Lightness constancy helps you recognize objects
under different lighting conditions. A piece of paper looks "white" whether it's
in bright sunlight or dim shadow, even though the light reaching your eyes differs enormously.
The paradox is that this helpful mechanism can be fooled—and when it is,
you have NO WAY to override it through willpower alone. Knowing the truth doesn't change what you see.