Thortz

Here’s an amazing thing: the squares marked A and B are the same shade of gray! Yes, that’s right. I’ll say it again more slowly: the darker-looking square marked A at the top of the board and the lighter-looking square marked B in the shadowy area are actually the same physical colour. Your monitor is displaying the same colour values in each area.

You can probably guess part of why the illusion works so well—we automatically adjust to counter the darkening due to the shadow —but the illusion continues working even when the green cylinder “causing” the shadow is cropped out:

This shows another part of why the illusion works; we make different assumptions about sharp edges than we do about gradients. The final picture should help you to see the truth of the claim that the squares are the same gray:

The illusion’s creator, Edward H Adelson, provides a more complete explanation.

It’s not new; I’ve been personally marvelling at it for a few years now. But familiarity doesn’t seem to defuse its effect. I consciously understand its workings, I can talk myself through it, but I just can’t make its reality seem intuitively present to me. It’s a fantastic optical illusion, but it’s also so much more; it’s a great scientific parable that illustrates key attributes of the way we think. Read more »