The Physics of Light and Color
Sir Isaac Newton, renowned for his law of universal gravitation, also investigated the nature of light. As early as 1672, Newton learned that white light can be divided into an orderly spectrum of colors by bending that light through a prism. Each color occurs as a specific wavelength of electromagnetic radiation. Every object reflects certain wavelengths of light and absorbs others. You perceive the differences in these reflected light waves as color. You are capable of distinguishing millions of different colors thanks to the cones of your retinas — each of which, in fact, is sensitive to one of only three colors.
Cones are present throughout the retina, but they are packed together most densely in the central area of the retina, which is called the fovea. Cones contain molecules called photopigments that absorb light and change shape, setting off a change in electrical state that is transmitted to the brain. Each cone contains one of three different versions of these photopigments, making it sensitive only to red, green or blue wavelengths. The interaction of the information transmitted by the eye’s cones and the brain creates the colors you see.
Each human eye contains about six million cones. The quantity you have of each type of cone and the quality of their functionality affect your ability to see subtle color variations. However, the brain relies also on comparisons between the input from different cones and between an object and the other parts of the image you are seeing. Researchers such as neurobiologist Semir Zeki, for example, study the regions of the brain specifically concerned with color constancy, that is, the ability of the brain to recognize and discount temporary lighting conditions that affect a color’s appearance in order to maintain a constant perception of color. Without this constancy, color would be useless as a means of identifying things.
Seven percent of American males suffer from varying degrees of color blindness, which is an inability to differentiate red from green. This trait originates during initial exchanges of genetic material once egg and sperm meet. Jeremy Nathans, a Howard Hughes Medical Institute investigator working at Johns Hopkins University School of Medicine, has found that the genes that create red and green color receptors are located near each other. Their DNA sequences differ by only 2 percent, making exchanges that result in absent or malfunctioning red or green cones more likely to occur. Only 0.4 percent of women are colorblind because women receive two copies of the X chromosome, which carries the genes for these two receptors. This reduces their odds of ending up with only defective versions of the genes. Blue color blindness is extremely rare; it is only caused by gene mutation.