Irradiation Illusion

The irradiation illusion is an illusion of visual perception in which a light-colored area of the visual field appears larger than an otherwise identical dark-colored area of exactly the same size and shape. The effect arises from light scattering inside the eye itself, which physically enlarges the image that a brighter area casts onto the retina compared with the image cast by an equally sized but darker area. The physicist and physiologist Hermann von Helmholtz gave the effect its name around 1867, though the underlying phenomenon had already been recognized much earlier, having been documented by Galileo Galilei in his 1632 work on the two competing systems of the world, in which he discussed how bright celestial objects appear larger against a dark sky than their true size would suggest.