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  CHAPTER 2

  Now You See It, Now You Don’t

  All forms of light, from the visible to the invisible, reside on the electromagnetic spectrum. Along this range there are many kinds (and colors) of light, and each variety can be distinguished from the others by two straightforward properties.

  The first is wavelength. The length of each individual light wave varies from the tiniest fraction of an inch to more than a mile and spans everything in between.

  The second is frequency, meaning the period of time it takes the wave to pass you and be replaced by the next wave, as if you were seated in a reviewing stand watching the light parade before your eyes.

  Think of an ocean wave. In the open sea, a typical wave is around one hundred yards, or ninety-one meters, long—roughly the length of a football field. Its frequency is a bit less than one second. This means that each wave’s peak requires nearly a second to pass any given point and be replaced by a trough, which in turn is followed by the peak of the next wave.

  Science can identify any wave, or any particular type of light, by either its length or its frequency. For example, each wave of green light at a traffic signal has a length of 530 nm (or nanometers, meaning 530 billionths of a meter), which is about one millionth of an inch. These tiny waves have a frequency of 530 THz, or terahertz, which means that 530 trillion of them pass your eye each second. (That the number 530 appears in both wavelength and frequency is a coincidence; the matchup is true of green light but not of any other color).

  When the signal turns red, you perceive waves of a longer length—twice as long, in fact. Each crimson-light wave is around two millionths of an inch from crest to crest. Red light has the longest waves of all visible light, but they’re still smaller than most germs in our body. These waves vibrate more slowly than green light, too, with “just” 450 trillion wave pulses occurring per second. What’s important is that all the light we see has wavelengths somewhere between 400 and 700 nm, which used to be expressed as 4,000–7,000 angstroms. The light we cannot see has wavelengths either shorter or longer than that.

  Short waves pulse, or change, more quickly than long ones, and this gives them more power, or energy. As a result, while the light we can see is too weak to break atoms apart, fast-vibrating light such as ultraviolet light can indeed strip an atom of one or more of its electrons, which alters molecules and can lead to consequences such as carcinogenesis.

  Invisible light has generally been named according to either its wave size or its position on the spectrum compared to the visible colors. Thus infrared light occupies a place just before the visible red light in the spectrum, meaning its waves are a bit longer than the red-light waves coming at you from a stoplight. By contrast, ultraviolet light lies just after violet light, and its waves are slightly shorter.

  The weakest kind of light is a radio wave. The longest radio waves measure a thousand miles from crest to crest. By contrast, the distance from one visible light wave to the next is just one millionth of a meter, or one hundred-thousandth of an inch. A few hundred trillion visible waves pass you every second. Even more mind-bogglingly short and fast are gamma rays, the strongest kind of light, with crests spaced just a trillionth of a meter apart and frequencies of a billion trillion per second. All other parts of the spectrum lie in between radio waves and gamma rays.

  Visible light occupies only a tiny part of the electromagnetic spectrum. (Wikimedia Commons)

  Except for the dim glow of the stars, all light is ultimately solar. Moonlight is reflected sunlight. The aurora borealis results from solar particles electrically stimulating the sparse oxygen atoms a hundred miles up. Candlelight and other kinds of flame require combustible materials such as coal, wood, and oil, which are forms of stored energy from long-dead plants and animals that would never have existed without the sun.

  In our era we also create light using electricity, but that, too, comes from burning oil, gas, coal, or hydropower generated by falling water, which would never circulate back to higher altitudes without everyday solar warmth. Only nuclear power and starlight are independent of the sun, and stars emit exactly the same visible and invisible rays as our own sun does. Stars differ only in their proportions: hot, massive stars emit copious ultraviolet rays and blue light, whereas the more numerous lightweight stars give off copious reds, oranges, and infrared radiation, with very few UV rays. Rather poetically, our eyes see only the colors the sun emits most strongly. Our retinas are designed to perceive sunlight’s most abundant energies and nothing else. So we really do have a sun bias. In a way, we scan the universe through the sun’s eyes.

  As we learned in fifth-grade earth science, the sun’s white light is merely our retinal and neurological response to receiving all the sun’s component spectral emissions at the same moment. White means we’re getting it all. In a very real sense, white is a rainbow in a blender.

  Indeed, if a scientist looks through a spectroscope, which reveals the true colors in the object she is studying by “unscrambling” them, a cloud that appears white to the unaided eye will resemble a vivid rainbow. The instrument reveals that a white cloud is actually composed of red, orange, yellow, green, blue, indigo, and violet light, and when those colors hit our eyes all at once, we see white. Studies conducted way back in the eighteenth century showed that even if a few of those colors are absent—orange and violet, say—we will still see white. Turns out all that’s needed to make white are blue, red, and green combined in equal measure. These are called light’s primary colors (totally different from yellow, cyan, and magenta, the primary colors of paint and pigment). So if we see white, it means we’re receiving red, blue, and green light simultaneously.

  If they’re combined unequally, those same three primary colors will create others. Your computer and your TV use this trick all the time. If a friend sends you a digital photo of autumn foliage, you might see that some leaves appear deep reddish-purple on the screen. Your computer creates this effect by mixing, say, eighteen parts blue, seven parts red, and one part green. Just three hues, commonly called RGB, for red, green, and blue, can combine to create every possible color.

  Some combinations are not logically obvious. Guess what mixture is needed to create yellow light? The answer is an equal blend of green and red. This surprises many people, because it seems logical that mixing red and green light would create a sort of reddish-green sensation. But no. Yellow is our eyes’ reaction to red mixed equally with green.

  On the other hand, if we mix red and green paint instead of red and green light, we won’t get yellow but rather some muddy brownish-black hue. That’s because entirely different rules apply to paint. Pigment does not glow on its own; we see a paint color only because some external white light (e.g., the ceiling lamp or the daylight streaming in from windows), is hitting the palette or painting, where the pigments absorb some of the white light’s colors and reflect others. Yellow paint, for example, looks yellow because its chemical absorbs white light’s blue component but reflects the light’s red and green components—and the combination of red and green light always yields the sensation of yellow.

  We see pigments and paints by a subtractive process. If you’ve ever painted, you may have had the frustrating experience of trying to create a color by mixing others—and ending up with a palette covered in brown. That’s because each new color you add introduces further subtractions from the white light illuminating the room, reducing reflections from the canvas so that less light reaches your eyes. Further pigment combinations invariably darken the image. Add too many pigments, and the result is muddy brown or black, because by that point nearly all light is absorbed by the paint’s molecules and nothing is reflected to our eyes. But light is a different story. Adding more light always brightens an image.

  The experience of vision is a symbiotic event. This is so fundamentally important yet so little known that it bears repeating from the previous chapter: by itself, light has no color or brightness. Light is merely a wave of magneti
sm and, at right angles to it, a wave of electricity. So the real external world is as utterly invisible as radio waves. Without humans to perceive it, “external reality” is nothing but a complex jumble of various blank energy frequencies. But when stimulated by these invisible frequencies, our six million cone-shaped, photon-sensing retinal cells respond, each to a rather narrow set of predetermined vibrations. Thus stimulated, they send an electrical signal at 250 miles per hour up the optic nerve until several hundred billion neurons in the rear of the brain fire in a continuous, complex way. The result is an image perceived in the brain as a color, such as blue.

  Bottom line: the “external world” is an internal experience. On their own, colors are not “out there.” When no one is looking, a sunset has neither color nor brightness. It is an invisible mélange of electrical and magnetic pulses.

  Some of us undergo an unusual subjective experience when confronted with the sun’s photons. Deuteranopes—the 10 percent of males who lack the green retinal receptor—see far fewer colors than the rest of us do. To them, shades of red and green can look identical—to each other as well as to what we perceive as yellow, the combination of the two. Theirs is a world of blues and yellows. They don’t understand why the rest of us are so enchanted by a rainbow, because to them it’s merely a swath of two colors. These people can easily run traffic lights if the bulbs are in unfamiliar positions. Turns out dogs and elephants are deuteranopes, too. It’s one of the reasons we should never let them drive.

  In sunlight or bright artificial illumination, our best retinal mechanism—the cone-shaped retinal cells that deliver full-color vision three times sharper than our prized 1080p high-definition TV—is able to operate. This is why we see best when we’re looking straight ahead: the densest concentration of cone cells lies in the center of the retina. The keenest vision also lies in the green part of the spectrum, right in the middle of our visual color range. Since green is our most readily perceived color as well as the one the sun emits most strongly, it perhaps deserves a few moments of our time.

  Our eyes can distinguish between wavelengths that differ by just one nanometer, but only in the green section of the color palette. When it comes to reds and violets, our visual acumen is only one-tenth that keen. This means that human vision can detect around fifty different shades of green when they’re positioned side by side.

  The test for color sensitivity is usually performed using a split screen on which slightly different wavelengths are displayed next to each other. When there is less than one nanometer difference between them, the observer reports a unity of appearance. As slightly different tints are displayed on the halves, a critical point arrives when a separate color is sensed: the screen suddenly seems sharply divided in two.

  Researchers test animals using the same setup, rewarding dogs if they push their noses onto the part of the screen where the color is different (and presumably wiping all those nose prints clean at some point). Similar studies show that cats see colors, too. Whether in man, monkey, or Maltese, all this color perception is called photopic vision. Photopic vision is the full-color, full-sharpness vision we enjoy whenever there is ample light.

  All this photopic vision from our bright-light neurological architecture has interesting idiosyncrasies. We already know that we see yellow when we receive an equal dose of green and red light. But this contrasts with our subjective experience of light’s other primary colors. We do indeed perceive reddish blue as the sensation of purple, and we apprehend green plus blue as aquamarine, or cyan, which does resemble both of its components. Yet thanks to the quirks of human eye-brain architecture, we cannot perceive reddish green or yellowish blue.

  Another oddity involves sensitivity. The daytime sky actually appears violet to a number of animal species, but our human retinas are so insensitive to the wavelengths at the fringe of the spectrum that we instead see the next most prevalent color emitted by the sky—blue. The animals and insects that can see the true violet color of the sky are also able to perceive ultraviolet light. Birds in particular readily see far more colors than we do. Their ability to perceive ultraviolet light allows them to detect the glow of mouse urine in a field far below.

  Yet even with all the vagaries of our photopic, or full-color, vision architecture, everything really changes when the sun sets. When the photon count declines, our eyes shift from normal photopic vision to scotopic vision, or night vision.

  Of all life’s solar-induced rhythms, this daily plunge into darkness is the most familiar. Yet how much do you know about the vision you’ve used every night of your life? As darkness descends, our pupils expand to triple their previous size—as much as seven or eight millimeters in diameter if we’re young. (Our pupils generally cannot expand beyond four or five millimeters after we pass the age of fifty.) Nineteenth-century astronomers, eager to be the first to detect new galaxies, or nebulae, would sometimes use belladonna to enlarge their pupils beyond their usual size in hopes of letting in more light and being able to see more through their telescopes.

  At the same time, in dim light, photochemical changes in the retina greatly boost its sensitivity. Before our scotopic vision has fully switched on—that is, in dim but not truly dark conditions, such as in twilight or in a room lit only by a night-light—photopic vision still operates, but not well. Only green objects keep their color, and the colors at the ends of the spectrum, red and violet, appear gray. We experience this in full moonlight, where everything in nature seems reduced to a single shade of aqua. At the same time, our peak sensitivity shifts from yellow-green to blue-green, a change first reported by nineteenth-century Czech physiologist Jan Purkinje, the guy who also first suggested that fingerprints might be useful in solving crimes. We still call that low-light alteration the Purkinje shift. This human sensitivity to green explains why the US interstate highway system, initially constructed in the 1950s, uses mostly green signs and why increasing numbers of municipalities now purchase green instead of red fire trucks.

  As light fades further, even green-blue hues vanish. Our eyes then work solely by their twenty million rod-shaped retinal cells. It’s as if our eyes have switched to a different kind of film. Our scotopic vision is dead-drunk slow to get going. Rod cells are lazy; they need repeated stimulation to operate at all. At night, when you turn off your bedroom lights, you first see nothing but total darkness. Within a few seconds some details slowly emerge. After five minutes the general features of the room are apparent. In twenty minutes, if you’re still awake, you see everything you’re ever going to see. But if someone clicks on the bright lights for a moment, then switches them off, you’re back to where you started, and you must begin the process of “dark adaptation” from scratch.

  Our scotopic vision is also color-blind. That red sweatshirt and those blue socks you threw over the chair—they’re gray now. The whole room is monochrome. (Incidentally, the only animals known to be totally blind to all colors are owls—suggested by the fact that their feathers are not vividly hued, a telltale sign that they have no need to use colorful plumage in order to attract the opposite sex—although their dim-light monochrome acuity is almost infinitely sharper than ours.) Unlike other colors, deep reds don’t just go gray in dim light; they vanish altogether. Rod cells simply cannot perceive wavelengths longer than 630 nanometers, which, unfortunately, includes the most common color in the universe, the red glow of excited hydrogen that is the calling card of giant interstellar gas clouds like the Orion nebula. One fascinating demonstration involves Christmas lights on a rheostat, or dimmer switch. As the brightness is turned way down, blue, yellow, orange, and green bulbs reach a dim point where they go gray. But deep red lights never turn gray. When they get sufficiently faint, they simply vanish.

  If you’re a human—a likely state of affairs if you’re reading this book—your night vision is very blurry. Normal vision is said to be 20/20 in bright light, although many young people could read the 20/10 line on the Snellen chart if only they were asked to try. (Tha
t’s line number 11, the bottommost.) But in dim light our best visual acuity is 20/200. That’s legally blind. If you’re going out with someone on a first date and are strolling along a dark street or in a park at night, you could play the sympathy angle by telling him or her that you are legally blind. You wouldn’t be lying.

  In dim light you have a blind spot in the very middle of your field of vision. It’s located dead center, it’s present in both eyes, and it’s large—twice the size of the full moon as it appears in the sky. This happens because only cone cells are located centrally, which is why we see best in bright light by looking directly at the object of interest. But at night we are best able to perceive faint details when looking slightly off to the side. Astronomers have known this for centuries. Most eyes can resolve the many individual stars in the Beehive star cluster in the constellation Cancer, but only by using averted vision. When stared at directly, the cluster is a blurry blob.

  Understanding scotopic, or faint-light, vision provides keys to understanding our blindness to many kinds of invisible light as well. Every night of our lives, we find ourselves blind to colors that we effortlessly see in the daylight. So when you’re thinking about the invisible part of the electromagnetic spectrum—radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays—you can put them in the same category as color frequencies that are undetectable to our night vision. They’re there, no less real than they are during the daytime; we just can’t see them. The reason is threefold.

  First, outside the visible wavelengths and infrared radiation (which we do perceive, or at least our skin does, as heat), we enter the realm of energies, such as microwaves, that the sun either does not emit at all or gives off very weakly. Why should our vision be able to locate objects only when microwaves bounce off them? The sun emits almost no microwaves, and thus these objects are not around us in nature. Why should we have a visual architecture that can detect things that aren’t there and that don’t affect us?