How Does It Work
The back of your eye is layered with special cells that respond to light. There are two kinds of these “photoreceptors” (light receivers) and they’re called rods and cones because of their shapes. It’s estimated that there are 120 million rod cells and a paltry 6-7 million cone cells in each of your eyes. Rods are more sensitive to light and dark changes so you use them more at night or in a dark space. Cones respond better in bright light to wavelengths of color so they work mostly during the daytime. Amazingly, like a TV or computer screen, there are three types of cone cells that are red-, green-, and blue-light sensitive. Your perception of colors comes from how these cells fire together and send the messages to your brain for interpretation.
When you stare at a color for a long time, the cones detecting that color get tired or fatigued and your eyes become less responsive to that color of light. After several seconds of staring and then looking away at a white surface, what you see is an afterimage that results from the photoreceptors not being balanced. The cells temporarily “subtract” cyan, black, and yellow light from the white light coming off the white page. The colors of the US flag occur because red, white, and blue are the complementary colors of cyan, black, and yellow light. After a few seconds, chemical balance is restored in the cells and the afterimage fades away.
