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Sometimes, you can’t believe your eyes. That’s because the human visual system is so sophisticated, and so good at showing your brain what’s out there, that a clever illusion can trip it up.

As one vision expert explains, “The visual system is not very good at being a ‘physical light meter,’ but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.”

The fellow who said that is Edward Adelson, Professor of Vision Science, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology.

Adelson’s department studies all aspects of visual perception, including illusions. Seeing how illusions fool the eye sheds light on how the visual system works. Puns intended.

Most of Alderson’s work makes sense only to his fellow brainiacs. For example, he said this in a paper on how we perceive surface reflectance:

“Human observers can distinguish the albedo of real-world surfaces even when the surfaces are viewed in isolation, contrary to the Gelb effect.”

See what I mean?

To demonstrate how illusions play their sneaky tricks, Alderson presents two fascinating examples that even a Journalism major like me can understand.

The first is the Checker Shadow Illusion.

In this image, the squares marked A and B are, in fact, the same shade of gray.

Not so, you reply. Square A is much darker than square B. Any fool can see that.

But it’s true. They are the exact same shade of gray. Even when you know the truth and concentrate vigorously, your eye still fools your brain.

To prove A and B match, Adelson modifies the image thusly:

Professor Adelson accepts your apology.

So, how does the checker shadow illusion fool us?

First, notice that square A is surrounded by lighter squares, and square B is surrounded by darker squares. In comparison, square A is made to look darker than it really is, and square B is made to appear lighter.

The shadow, given emphasis by the looming object that casts it, is a further complication. Your visual system does not like shadows. It knows that shadows can be misleading. It is distracted, confused, misled.

The second illusion is the Munker-White Illusion.

In these three figures, you perceive two shades of each color, a light shade and a dark shade.

That’s an illusion. In truth, all gray bars are the same shade of gray, all blue bars are the same shade of blue, and all red bars are the same shade of red.

This illusion uses the placement of the black and white bands to fool your visual system. The bars next to the black bands appear darker. The bars next to the white bands appear lighter.

Consider this variation showing the blue bars only.

On the left side, the blue bars are depicted with white bands above and below. On the right side, the blue bars are shown with black bands above and below. The appearance of two shades of blue is quite clear.

However, removing the bands reveals that both sets of blue bars are the same shade.

Sometimes, you can’t believe your eyes.