17-1 What are the characteristics of the energy that we see as visible light? What structures in the eye help focus that energy?

Our eyes receive light energy and transduce (transform) it into neural messages that our brain then processes into what we consciously see. How does such a taken-for-granted yet extraordinary thing happen?

When you look at a bright red tulip, the stimuli striking your eyes are not particles of the color red but pulses of electromagnetic energy that your visual system perceives as red. What we see as visible light is but a thin slice of the whole spectrum of electromagnetic energy, ranging from imperceptibly short gamma waves to the long waves of radio transmission (FIGURE 17.1 below). Other organisms are sensitive to differing portions of the spectrum. Bees, for instance, cannot see what we perceive as red but can see ultraviolet light.

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Two physical characteristics of light help determine our experience. Light’s wavelength—the distance from one wave peak to the next (FIGURE 17.2a below)—determines its hue (the color we experience, such as a tulip’s red petals or green leaves). Intensity—the amount of energy in light waves (determined by a wave’s amplitude, or height)—influences brightness (FIGURE 17.2b). To understand how we transform physical energy into color and meaning, consider the eye.

Light enters the eye through the cornea, which bends light to help provide focus (FIGURE 17.3 below). The light then passes through the pupil, a small adjustable opening surrounded by the iris, a colored muscle that controls the size of the pupil by dilating or constricting in response to light intensity—or even to imagining a sunny sky or a dark room (Laeng & Sulutvedt, 2014). The iris also responds to our cognitive and emotional states. When you feel disgust or you are about to answer No to a question, your pupils constrict (de Gee et al., 2014; Goldinger & Papesh, 2012). When you’re feeling amorous, your telltale dilated pupils and resulting dark eyes subtly signal your interest. Each iris is so distinctive that an iris-scanning machine can confirm your identity.

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Behind the pupil is a transparent lens that focuses incoming light rays into an image on the retina, a multilayered tissue on the eyeball’s sensitive inner surface. The lens focuses the rays by changing its curvature and thickness, in a process called accommodation.

For centuries, scientists knew that when an image of a candle passes through a small opening, it casts an inverted mirror image on a dark wall behind. If the image passing through the pupil casts this sort of upside-down image on the retina, as in FIGURE 17.3, how can we see the world right side up? Eventually, the answer became clear: The retina doesn’t “see” a whole image. Rather, its millions of receptor cells convert particles of light energy into neural impulses and forward those to the brain. There, the impulses are reassembled into a perceived, upright-seeming image. And along the way, visual information processing percolates through progressively more abstract levels, all at astonishing speed.