BLUE SHADOWS - PHYSICAL, PHYSIOLOGICAL, AND PSYCHOLOGICAL CAUSES

The apparent blueness of outdoor shadows has two main causes: the illu mination of the shadows by blue skylight and the enhancement of the pe rception of blue by simultaneous color contrast. Other physiological m echanisms, such as brightness contrast and afterimages, can also affec t the perception of a shadow's blueness. Preferential scattering by th e cornea does not seem to make a major contribution. Despite these eff ects, color constancy causes most people to observe an empirically blu e shadow as colorless.