Our brain filters and interprets what we see to help make sense of the World around us. Simply measuring the light coming from a surface is not enough to understand the surface. A white surface in shadow, for example, may reflect less light than a black surface in full day light.
I first came across Edward H. Adelson’s maddening checker shadow optical illusion while reading Kathryn Schultz’s fascinating book “Being Wrong”. The squares labelled A and B in the following picture are the exact same shade.
You don’t have to take my word for it, you can watch it on YouTube – in all its maddening glory!
This process appears hardwired. It doesn’t matter how many times you see this illusion, your brain still gets it wrong!
I recommend the book. It’s rather humbling to realise how wrong you can be; from what you hear, what you see and what you think!
Thanks to Sam Owen telling about the YouTube video.