Richard Dawkins, the evolutionary biologist, is well known for holding no punches. His attacks on those with whom he disagrees are often scathing. As he is well aware, there are many who think him arrogant and obnoxious. (Frankly, count me among them.) However, I’m grateful for what I’ve learned from him, including the insight which forms the basis of today’s blog. It comes from his book, Unweaving the Rainbow.
Thanks to the Biblical story of Noah, many of us associate the rainbow with hope. Count me among those who see great value in hope. But how good is our understanding of what rainbows are in a physical sense? Everyone knows you can’t really find pots of gold at their ends, and that they’re really some sort of apparition or mirage – that you can’t get close enough to touch one, or it will disappear. But what are rainbows — really?
Not only can’t we touch them, we don’t generally hear, smell, or taste them, either. We might all agree that they are some sort of purely visual phenomenon. We know what one is, we can all agree on what one is, because when you see one, I likely do too, so we know we’re talking about the same thing, right?
According to Dawkins, wrong.
At this point, please forgive me if I describe what you already know, but for a moment, permit me to summarize some high school learning about light. First, if you thought that light travels at about 186,000 miles per second – the “c” in Einstein’s famous E=mc2 – you’d be wrong, since that’s only the speed at which light travels in a vacuum. In fact, when not in a vacuum, light travels at different speeds depending on what medium it’s passing through, and that’s why light speeds up or slows down when it passes from one medium to another. And that, in turn, is why the light that hits a lens gets refracted (bent) – because it’s passing from one medium (air) into another (glass). And it gets refracted a second time when it leaves the lens, passing from glass back into the air. The stratified bands of light we see as a rainbow result when the white light from the sun – white being a mixture of all the wavelengths of visible light — is variously refracted (bent) into the rainbow’s color bands, much the same way the white light from Isaac Newton’s window was bent by a prism to produce separate bands of different colors. Dawkins: “The prism sorts them out by bending them through different angles, blue through a steeper angle than red; green, yellow and orange through intermediate angles.” Many of us were taught this in high school. No big surprise there.
As Dawkins points out, the little lenses that are refracting the light we see as rainbows are individual drops of rain. And because the rain drops are more or less spherical, their surfaces not only refract (bend) the light, they also reflect it (like mirrors). So in a sense, we’re not really looking at the rainbow at all, but at a mirror image of it – a reflection of it. But wait, like the commercials say. There’s more.
Every single raindrop is refracting and reflecting the sunlight that hits it, breaking that light into its component colors, and bending them differently, sending them out at different angles, the reds in this direction, the blues in that, and so on. Dawson again: “If your eye gets a beam of green light from one particular raindrop, the blue light from that raindrop goes above your eye, and the red light from that particular raindrop goes below. So, why do you see a complete rainbow? Because there are lots of different raindrops. A band of thousands of raindrops is giving you green light (and simultaneously giving blue light to anybody who might be suitably placed above you, and simultaneously giving red light to somebody else below you).”
So in fact, if you climbed a ladder, while you’d still have the sensation of seeing the same rainbow, you’d actually be seeing a very different set of raindrops – or, more precisely, waves of light being refracted and reflected from a very different set of raindrops. Moreover, Dawkins goes on to explain that the reason the rainbows all look curved is that the raindrops giving you the same color sensation are all at a fixed distance from you. (Imagine you’re in your high school geometry class, and you’re the point of a compass stuck fast into your graph paper, not moving, while the pencil point at the other side of the compass “describes an arc” or smooth curve around you. A similar arc-shaped pencil line in the atmosphere is where the rain drops giving you the sensation of red are. And a slightly different arc-shaped pencil line contains the raindrops sending you the sensation of blue.) But whatever the color band, these sets of raindrops are all at fixed distances from your eyes, which is why you are always at the center of the raindrops you perceive – and why rainbows always form a curve, precisely, with you at the center.
If by “rainbow,” then, we don’t mean a bunch of transparent raindrops in the sky, but an arc-shaped spectral band of color, it follows that a rainbow – like all beauty – lies in the eyes of the beholder. Quite literally, if you move just a step or two in a different direction, you’ll be seeing a different set of raindrops – a different rainbow from anyone else around you, and one that only becomes “real” once it’s inside your eyes.
That brings me to some final observations. Traditional thinking has long been that the colors we perceive result from the fact that we human beings have three types of cone cell in our retinas, one of which generally responds to short wavelengths of light, one to medium wavelengths, and one to long wavelengths. Yet recent discoveries show that there are some people, called tetrachromats, who possess a fourth type of cone, who actually see color differently as a result of that biological difference. And then there are those we call “color blind” who lack one of the three types that most of us have. So what you see as the rainbow varies, depending on whether you have two, three, or four types of cones in your retinas. There are apparently numerous other differences in how the eyes of different individuals perceive the same set of light waves as “color,” some studies even suggesting that, in the same individual, color sensation may be affected by mood. So the rainbow you see on a happy day may be different from what you’d see if you were sad; and it may certainly be different from the rainbow your neighbor would perceive, even if, somehow, you were occupying precisely the same point in space at the same point in time – even if the same light waves were hitting both sets of eyes at once.
Back in the day, we might have said that most of us perceive the rainbow as it really is – those unfortunate souls we call “color-blind” don’t see things “correctly.” Nowadays, we have to deal with the tetrachromats. If they see more color variation than we do, is it in fact we who are “color blind”?
Even if we consider only the “typical” human being on a “typical” day, we are all only capable of perceiving “visible” light. We think of the word “visible” as meaning “capable of being seen.” But what we really mean is not the capacity to be seen by anyone or by anything – we really mean the capacity to be seen by most human beings. Some snakes can see infrared wavelengths which we cannot. To quote Dawkins again, “There is nothing special about the narrow band of wavelengths that we call light, apart from the fact that we can see it. For insects, visible light is shifted bodily along the spectrum. Ultraviolet is for them a visible colour (‘bee purple’), and they are blind to red (which they might call ‘infra yellow’).”
Goldfish can see the ultraviolet light that we cannot. Most birds are tetrachromats. In other words, we humans have appropriated to ourselves the notion of “visible light” by defining it in terms of our own biological capacity to “see” (or not), when in fact, there is far more out there which other species can see, but we cannot. Imagine how confused we might be if, like Superman, we had “x-ray vision” that mixed our perception of “visible light” with images, not of fully clothed neighbors, but of their skeletons as well? Imagine how much larger our brains would have to be if, instead of there being just tetrachromats among us, we all had a thousand types of cones in our retinas, and we all had the resulting tons of additional information to process.
In my view, we’d do well to reflect on the fact that we’ve been engineered — or have evolved, or whatever — to have limited sight — a capacity to see only the tiniest part of what is “real.” If what we “see” is different from what snakes, insects, birds or goldfish see, which vision is the “correct” one? And if the rainbow I see is different than the one you see, can either of us be certain we’re right, or that the other is wrong?