PHYSICS AND CHEMISTRY
Plato, Newton, Da Vinci, Goethe, Einstein: All these great minds and many more grappled with the profound complexity of color. They sought to understand it, creating systems to explain its mysterious workings.
Some fared better than others, and from the vantage point of our current scientific knowledge, many of their attempts now seem funny, bizarre, or downright fantastic. In the fifth century BC, Plato drew a causal relationship between color vision and the tears in our eyes. The eighteenth—and nineteenth-century philosopher Johann Wolfgang von Goethe tried to impose order on color's chaos by arranging hues into three groups: powerful, gentle/soft, and radiant/splendid. Although we've come a long way in our understanding of color, much remains a mystery.
Color is everywhere, but most of us never think to ask about its origins. The average person has no idea why the sky is blue, the grass green, the rose red. We take such things for granted. But the sky is not blue, the grass is not green, the rose is not red. It has taken us centuries to figure this out.
In a very dark chamber, at a round hole, about one-third part of an inch broad, made in the shut of a window. I placed a glass prism, whereby the beam of the Sun's light, which came in that hole, might be refracted upwards toward the opposite wall of the chamber, and there form a colored image of the sun.
—Sir Isaac Newton, Opticks
It stands to reason that for thousands of years, many casual observers must have seen what Newton did: that light passing through a prism creates a rainbow on the surface where it lands; but Newton saw something no one else had seen. He deduced that the white light that appears to surround us actually contains all the different colors we find in a rainbow. White was not separate from these colors—or a color unto itself—but was the result of all colors being reflected at once. This counterintuitive and revolutionary theory did not take hold easily. Some of those greatest minds we mentioned simply wouldn't accept this theory. The idea that white light contained all color upset Goethe so that he refused—and demanded others refuse—even to attempt Newton's experiment.
NEWTON'S PRISM Although Newton's discovery was more than enough to upset his contemporaries' digestion, he didn't stop there; Newton also ascertained that colors refracted through a prism could not be changed into other colors. Here's how he did it. He took a prism and placed it between a beam of light (coming from the hole in his window shutters) and a board with a small hole in it. The hole in the board was small enough that it only allowed one of the refracted colors to pass through it. He then placed all kinds of materials (including a second prism) in front of the beam passing through the small hole to try to alter the refracted color passing through the small hole. Prior to the experiment, he had believed that if, for example, a blue piece of glass was placed in front of a red beam of light, the red would be transformed into another color. But he found that this was not the case. No matter what color or type of material he placed in front of an individual beam of light, he couldn't get the refracted color to change. From this experiment, he deduced that there was a certain number of what he called "spectral" colors—colors that cannot be broken down, colors that are fundamental.
Once Newton confirmed that his spectral colors were unchangeable, he decided to name them—and here's where his method takes a left turn from the scientific to the fanciful. Taken with the idea that the rainbow should reflect the musical scale, Newton decided to name his colors in accordance with aesthetics. There are seven main tones in the musical scale, so Newton came up with seven corresponding colors. Hence the origin of ROYGBIV, the acronym by which we know Newton's seven spectral colors—red, orange, yellow, green, blue, indigo, and violet. Although the relationship to music was later set aside by scientists who questioned the basis for comparison, ROYGBIV is still used today as a teaching tool, even though indigo is not a color most people can even identify.
The truth is, there's no perfect way to name the colors of the rainbow. Take a look at a real rainbow (as opposed to a kindergartner's felt-tip rendition), and you'll see that its colors merge seamlessly from one to the other. Any judgment on where one color ends and the other begins is arbitrary. Even Newton waffled on this point. At the beginning of his experimentation, his spectrum included eleven colors. Once he'd whittled the number down to seven, he still thought of orange and indigo as less important, calling them semitones in another nod to the musical scale.
There's another issue with naming the colors of the rainbow: The language of color is fluid, morphing over time and across geographies in response to cultural forces that are sometimes too complex to pin down. For example, the color Newton called indigo is the one most people would identify as plain old blue or a true blue that falls midway between green and violet. Newton's blue is what we now call cyan, a more turquoise blue that falls between blue and green.
As for the name of the last color in the rainbow, why is it violet as opposed to purple? Violet refers to the spectral color that looks bluish purple. Purple refers not to a spectral color but to a color created by a mix of light.
Color systems pre-and post-Newton have codified a whole host of fundamental colors. By fundamental, we mean colors that our linguistic or scientific models don't allow us to reduce any further. Today, if you were asked to categorize the color navy, you would probably call it a dark blue. If forced to generalize it even further, you would just say it's a shade of blue, but there's nowhere to go from there. Throughout history, our cultural understanding of fundamental colors has shifted dramatically, ranging from the stark black and white model (where colors were categorized merely by how dark or light they were) to systems made up of dozens of colors, to systems in which some combination of red, yellow, blue, green, and sometimes orange and violet/purple typically reigns.
Today, we consider red, orange, yellow, green, blue, and violet as fundamental. These fundamental colors are referred to as hues. Although a color's value (put simply, how dark or light it is) and its chroma (put simply, how dull or bright it is) can change, its hue is essential to its identification.
Regardless of their number, the colors Newton called spectral should not be confused with the colors we've been taught are the primary colors that cannot be mixed and are therefore fundamental (i.e., red, blue, and yellow) or the class of shades known as secondary colors (i.e. orange, green, and purple). These secondary colors, we've been taught, result from the mixing of the primary colors—red and yellow make orange, red and blue make purple, and blue and yellow make green— and therefore are not fundamental. But what Newton found was that orange, green, and violet (which, to repeat, differs from purple) can be spectral and just as fundamental as the colors we call primary. Orange, for example, can result from a mix of light, but it can also be pure. The same is true for red, blue, and yellow, even though we call them "primaries." You can tell a color that is a mixture of light from a spectral color by passing the light through a prism. The orange light that is a mix will break into its components when passed through a prism, but pure orange light will not.
Another major "aha moment" would soon follow for Newton: the discovery that when red light was passed through a prism, it bent only slightly, whereas violet light bent much more. This intriguing observation led Newton to believe that each color was made up of unique essential components. What made red red is different from what made violet violet. Although he was on the right track, Newton incorrectly hypothesized that light is composed of particles that travel in a straight line through some kind of ether. What he called his "particle theory" was eventually widely accepted.
Fast forward to the beginning of the nineteenth century, when an English scientist named Thomas Young returned to a notion put forth by some of Newton's contemporaries. Although Newton was convinced that light is a particle, Young's experiments led him to believe that light—like sound—is a wave. Another half-century later, the venerable James Clerk Maxwell took Young's work and made a giant leap.
JAMES CLERK MAXWELL AND THE REIGN OF ELECTROMAGNETISM Before James Clerk Maxwell's work, electricity and magnetism were believed to be two separate forces, but Maxwell found the forces to be connected, and he called this interconnection electromagnetism. Maxwell showed how charged particles repel or attract one another, as well as how these charged particles act like waves as they travel through space.
A particularly exciting part of Maxwell's treatise on the subject showed that a specific group of electromagnetic waves is the cause of visible light—in other words, the cause of color. He identified other groups of electromagnetic waves, too, groups we now recognize as ultraviolet light, radio waves, x-rays, and microwaves, to name a few. All of these fall on the electromagnetic spectrum, and each is measured and defined by its length and frequency, which are inversely proportional. Every color has a different length and frequency, as does every microwave, radio wave, or other wavelength on the electromagnetic spectrum; but there is no essential quality separating visible light from these other kinds of waves—except that our eyes (or to be more precise, our brains) can perceive them as color.
To get a handle on exactly what length and frequency are all about, imagine you are holding one end of a jump rope. Another person stands a few feet away holding the other end. If you move your hand up and down slowly, the movement will create one big arc—or wave—in the center of the jump rope. Move your hand a little faster, and you will get several waves occupying the same space as the single, big wave. Move your hand faster still, and you will get many more waves that are even closer together. The distance between the peak of one wave and the peak of the next wave is the wavelength. The frequency is the number of waves per second. As demonstrated via the jump rope, the smaller the wavelength, the higher the frequency (or number) of waves.
Violet is the color with the shortest wavelength of visible light, at 380 to 450 nanometers (one nanometer is equal to one-billionth of a meter), but the highest frequency at 789 to 668 THz (or terahertz, which is the unit for frequency). On the spectrum of electromagnetic radiation, it is the wave that is closest to ultraviolet and x-rays. Red has the longest wavelength, at 620 to 740 nanometers, but the lowest frequency of visible light is at 480 to 400 THz, closest to infrared and microwaves.
Once Maxwell had shown visible light to be just one piece of the electromagnetic puzzle, other pieces of the puzzle started falling into place—or falling out of place. The scientist Max Planck remained dissatisfied by Maxwell's theory that light was just a wave. His experiments pointed to another dimension of light that he couldn't quite put his finger on. Enter Albert Einstein, who took the reins and eventually settled on the idea that light is indeed not just a wave, but also a particle.
The fact that light can act at times like waves and at times like particles—a phenomenon called the wave-particle duality of nature—defies common sense, but it is the best explanation scientists have found. The wave-particle duality of nature led to quantum mechanics, which of all the branches of physics has arguably had, directly or indirectly, the most influence on our current understanding of the origins and workings of the universe.
THE REAL PRIMARY COLORS In addition to determining the physical properties of color, scientists began to ask questions about our perception of color. Why and how do we see color? The answers to these questions lead us down a path that will counter almost everything you've ever learned about color. Take the "white" light Newton was passing through his prism. Newton's conclusion was that all the colors of the rainbow combined to create white. But if you mix red, orange, yellow, green, blue, and violet paint in your palette, you get anything but white. (This, by the way, was the source of Goethe's extreme skepticism about Newton's theories—he had watched painters mix paint in their palettes and had never seen a multitude of colors add up to white.)
Newton was not dealing with paint, however; he was dealing with light, and light mixes in an entirely different fashion. The mixing of light belongs to the realm of additive color. When you add different wavelengths of light together, you don't get the muddy mess you see with paint and the mix doesn't always produce what you'd expect it to because, again, additive color doesn't work like paint. In fact, every color of the rainbow can be achieved by mixing only three colors of light: red, green and blue. Wait, green?
That's a different triad from the primary colors—the colors from which all others can theoretically be mixed—that we are taught about in school, namely, red, blue, and yellow. Where did green come from? And where did yellow go? These three new primary colors—red, green, and blue—make no sense from a young painter's perspective. Anyone who has ever mixed paint knows that you can't make yellow from any of these colors. Yet, when you mix what appears to our human eyes as red and green light, you do indeed get yellow.
You are probably more familiar than you think with this primary triad. Case in point: the RGB (red, green, blue) color model on your computer monitor. Obviously you see a wide range of colors on your screen, but if you got close to an old computer with visible pixils, you could actually see that the screen is entirely made up of red, green, and blue dots. The same is still true today; the dots are just harder to see. If you took a magnifying glass to your screen, you'd see that magenta type, a field of pumpkins, and a dull brown bunny were all composed of these red, green, and blue dots. A white screen is the result of all three colors lighting up at the same maximum intensity, and a black screen is the result of the absence of color.
Televisions and cameras also use an RGB color model, as does theater lighting. Similarly, the first color photography (a feat conducted by none other than James Clerk Maxwell in 1861) was a product of the layering of red-filtered, green-filtered, and blue-filtered negatives on top of one another. Here again, you can see the logical underpinnings of the term additive color whereby adding colors together can beget every color of the rainbow.
Why just red, green, and blue? What was the significance of these three colors? At the beginning of the nineteenth century, Thomas Young (the physicist who came up with the wave theory of light) wanted to find out. His answer would come from a study of the human eye.
YOUR BRAIN ON COLOR If there's one color-related axiom that bears repeating, it is this: Wavelengths of light do not exist as color until we see them. Without the eyes and brain, there's no such thing as color. Light waves are colorless until the moment they hit our eyes, at which point our brains declare, "Blue sky, green grass, red rose!" Most other animals—and even some humans—won't see any of these colors when they look at the sky, the grass, or a rose because, again, none of these entities is intrinsically colored.