I’m sure you’ve come across one of these optical illusions. On the image above the orange square (on the dark front side) and the brown square on the top are actually the same color. Hard to believe but true. For non-believers there are many ways to validate it. You can download a free color picker and read the values from the screen. You can cut those pieces with the help of an ordinary image editor, put on plain background and see or just print it, cut those portions and compare.
I’ve recently come across this flash based site, eChalk, that includes many of those optical illusions. They actually have a mask utility which can do the job of separating the necessary bits from the rest and make you see the truth.
Having opened the subject why not include some more sites.
Optical Illusions & Visual Phenomena: Excellent site on optical illusions. Also included is the theory behind these illusions. A must see.
Akiyoshi’s Illusion Pages: Includes some new illusions.
Mighty Optical Illusions: There are some really interesting stuff here.
Optical Illusions: Worth a visit. You may come across something new.
Color Vision & Art: Excellent site. Lots of information on color and the theory behind it. If you check out the whole site you’ll know the real reason behind many of those illusions. They’ve included many online sliders to make things clearer. Don’t forget to check out Claude Monet’s Impression Sunrise.