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In color theory, two colors are called complementary if, when mixed in the proper proportion, they produce a neutral color. When one stares at a color for a sustained period of time (roughly thirty seconds), then looks at a white surface, an afterimage of its complementary color will appear, an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased. This is one of several aftereffects studied in the psychology of visual perception which are generally ascribed to fatigue in specific parts of the visual system (1)
OBS.: In the next image, Keep looking continuosly at the gray dot in the center of the wheel to see the complementary afterimage phenomenon.