The Koffka Ring Illusion (Video Below)
Kurt Koffka was a Gestalt psychologist who studied the importance of grouping in perceptual phenomena.
In the case of lightness, he showed that a gray ring on a light/dark background looks uniform, but when the two halves are split, as in the middle image, they look quite different.
A new variant is shown at the bottom, where the halves are slid vertically, giving an impression of transparency. Even though the half-rings are set against the same backgrounds in each case, their appearance depends on the overall spatial configuration.
Press Play below to view the illusion.
This video demonstration of the Koffka Ring illusion was salvaged from the .swf file at http://web.mit.edu/persci/gaz/gaz-teaching/flash/koffka-movie.swf and converted into a web-friendly video format.
Copyright remains with the original creator, and is displayed here for educational purposes only. Adobe discontinued Flash at the end of 2020.
The original flash movie and others are based on a paper by Edward H. Adelson and can be found on MIT’s website here.