Koffka Ring

The Koffka ring is a geometric optical illusion, named for the German psychologist Kurt Koffka, consisting of a single ring of uniform gray color whose lower half is set against a black background and whose upper half is set against a white background. When a thin black dividing line separates the gray ring from the background behind it, the upper half of the ring, seen against the white background, appears noticeably darker than the lower half, seen against the black background, even though the entire ring is actually one uniform shade of gray throughout; removing the dividing line between the ring and its background causes this apparent contrast between the two halves to nearly disappear. The illusion is explained as an amplification, by the visual system, of the brightness difference between adjacent surfaces, an effect that grows stronger the more sharply the two background regions are separated from one another, and that becomes especially noticeable once the two halves of the ring are fully divided by the dividing line.