Cognitive scientist/psychologist Tom Stafford created this powerful example of chromostereopsis, a fantastic optical illusion where depth is delivered just by using two colors at opposite ends of the light spectrum. From Mind Hacks:
There are big individual differences in perception of the effect. This isn't just in terms of strength, although obviously I'm one of those it hits hard. People also differ in which colour looks closer. For most people it is red, with blue looking deeper or further away. I'm in the minority, so if you're like me this reverse of the image above should look more natural: the pupil set deeper than the surrounding eye[…]
The way chromostereopsis works is not entirely understood. Even the great Michael Bach, who wrote for the Mind Hacks book, describes the explanation for the phenomenon as 'multi-varied and intricate'. That red and blue are at opposite ends of the light spectrum has something to do with it, and the consequent fact that different wavelengths of light will be focussed differently on the back of the eyes. This may also be why some people report that their glasses intensify the effect. The luminance of the image and the background also seems to be important.
