Chromostereopsis

Chromostereopsis is a visual illusion in which a two-dimensional color image, usually made up of red and blue or red and green, though red-gray or blue-gray combinations can also work, conveys a false impression of depth. Reports of the illusion go back more than a century, and it is generally attributed to chromatic aberration, the tendency of light to refract by different amounts depending on its wavelength, which causes some wavelengths to come into focus before others within the eye and to land on slightly different, non-corresponding spots on the two eyes' retinas during binocular viewing. The illusion is usually demonstrated with a target made of red and blue bars set against a neutral, colorless background: positive chromostereopsis occurs when the red bars appear to float in front of the blue bars, and negative chromostereopsis occurs when the red bars appear to sit behind the blue ones, an effect that can appear considerably stronger for people wearing eyeglasses that correct nearsightedness. It has also been proposed that chromostereopsis carries evolutionary significance in the development of eyespot patterns seen in certain butterfly species.