5 Steps to a 5 AP Psychology, 2010-2011 Edition - Laura Lincoln Maitland [51]
Figure 8.1 The eye.
Color Vision
The colors of objects you see depend on the wavelengths of light reflected from those objects to your eyes. Light is the visible portion of the electromagnetic spectrum. Do you remember ROYGBIV? The letters stand for the colors red, orange, yellow, green, blue, indigo, and violet, which combine to produce white light. The colors vary in wavelength from the longest (red) to the shortest (violet). A wavelength is the distance from the top of one wave to the top of the next wave. The sun and most electric light bulbs essentially give off white light. When light hits an object, different wavelengths of light can be reflected, transmitted, or absorbed. Generally, the more lightwaves your eyes receive (the higher the amplitude of the wave), the brighter an object appears. The wavelengths of light that reach your eye from the object determine the color, or hue, the object appears to be. If an object absorbs all of the wavelengths, then none reach your eyes and the object appears black. If the object reflects all of the wavelengths, then all reach your eyes and the object appears white. If it absorbs some of the wavelengths and reflects others, the color you see results from the color(s) of the waves reflected. For example, a rose appears red when it absorbs orange, yellow, green, blue, indigo, and violet wavelengths and reflects the longer red wavelengths to your eyes.
“Information about colors of light is easy to remember: COlor receptors are COnes. ROYGBIV is a long acronym, so red is a long wave. Primary colors of light and the three kinds of cones are like my computer monitor and color TV—RGB.”—Matt, AP student
What enables you to perceive color? In the 1800s, Thomas Young and Hermann von Helmholtz accounted for color vision with the trichromatic theory that three different types of photoreceptors are each most sensitive to a different range of wavelengths. People with three different types of cones are called trichromats; with two different types, dichro-mats; and with only one, monochromats. Cones are maximally sensitive to red, green, or blue. Each color you see results from a specific ratio of activation among the three types of receptors. For example, yellow results from stimulation of red and green cones. People who are colorblind lack a chemical usually produced by one or more types of cones. The most common type of color blindness is red–green color blindness resulting from a defective gene on the X-chromosome, for a green cone chemical, or, less often, for a red cone chemical. Because it is a sex-linked recessive trait, males more frequently have this inability to distinguish colors in the red–orange–green range. Blue–yellow color blindness and total color blindness are rarer. Although trichromatic theory successfully accounts for how you can see any color in the spectrum, it cannot explain how mixing complementary colors produces the sensation of white, or why after staring at a red image, if you look at a white surface, you see green (a negative afterimage). According to Ewald Hering’s opponent-process theory, certain neurons can be either excited or inhibited, depending on the wavelength of light, and complementary wavelengths have opposite effects. For example, the ability to see reds and greens is mediated by red–green opponent cells, which are excited by wavelengths in the red area of the spectrum and inhibited by wavelengths in the green area of the spectrum, or vice versa. The ability to see blues and yellows is similar. Black–white opponent cells determine overall brightness. This explains why mixing complementary colors red and green or blue and yellow produces the perception of white, and the appearance of negative afterimages. Colors in afterimages are the complements of