Color is one of the principle elements of the visual arts. It is useful for artists to understand how colors are formed as well as how they are perceived. For the purposes of this class we will look first at color theory and then use this knowledge to explore why one chemical compound will give one color while a second compound generates another color. We will also look briefly at spectroscopy, a means of looking at the variations of intensities in light of different wavelengths across the whole spectrum of visible light. Here we will pay particular attention to the wavelengths of light that have been removed from the overall spectrum by interaction with matter. Chemists can use this technique as a way of determining which compounds are present, as a way of investigating the structure of compounds that are present, or as a means to determine how much of a particular compound is present. This technique can and will be used by us to look at a variety of pigments in egg tempera paints when we make color wheels in our class-wide experiment.


We begin by asking, "What happens when we see? What physics events are involved?" These are a ancient questions which philosophers such as Aristotle, Ptolemy and Galen tried to addressed. The common viewpoint passed down to European thinkers by medival times held that rays of came out of the viewer's eye, and when these rays hit an object, they would send back the image to the viewer's eye. But Europe was far behind the Islamic World in scintific thought and technology.

The 10th Century Arab scientist-philosopher Al Hazen found that the common view, the one one held in Europe, was preposterous. He developed an alternative theory in which light traveled in straight lines and that the light emanated from sources such as the sun or a candle instead of the viewer's eye. When this light hit an object the reflected rays carried its image to the viewer's eye. When these rays entered the pupils of the eye, they are focused as the point of a visual cone. This idea was adopted in the Renaissance by artists such as Brunellschi who developed the principles of perspective. More importantly for our discussion of color, Al Hazen investigated the origins of the rainbow using glass spheres filled with water. He found that as the light was refracted by the water, the angle of refraction of separate rays of various components of the light were different. Rays of red light were bent least and blue rays of light were bent most. Thus a "spectrum" of different colored lights that matched the rainbow was produced.


We'll jump ahead a few centuries. At the age of 23, Isaac Newton reinvestigated this same dispersion of white sunlight into a rainbow of colors. Newton had quarantined himself in his rooms to avoid the plague that was raging through England at the time. When he held a prism of glass in the path of a beam of sunlight coming through a hole in the blind of his darkened room, he observed that the white sunlight was split into red, orange, yellow, green, cyan and blue light. But Newton observed something no one else had because he extended the experiment. Using prisms and mirrors, he discovered that when the light from three separate parts of his rainbow, the red, green, and blue regions, were recombined they would regenerate white light. He called these the primary colors. When any two of these were combined, secondary colors were formed. When he combined blue and green light, he observed light the color of cyan. Green and red light mixed to give yellow light. In both of these cases, Newton apparently regenerated light in another portion of the natural spectrum. But when he combined red and blue light from his prisms, Newton observed a colored light, magenta, that was not found in the natural visible spectrum. Newton organized his findings in a color wheel showing the three "primary colors" -- red, green, and blue -- separated by the three "secondary colors" -- yellow, cyan, and magenta. Since magenta was a non-spectral color of light, its origins posed a mystery.
newton's wheel


In 1802, Thomas Young, an English physician who had a burning desire to understand the nature of light, made progress in solving this magenta puzzle. Young first demonstrated that he could generate any colors that could be seen by mixing differing proportions of the three primary colors of light. For example, you could mix two parts of red light with one part of green light to get an orange color. Using more green light than red light, you saw a yellow-green light. Young took his observations a step further: he hypothesized that the human eye perceives only Newton's three primary colors, red, green, and blue, and that the eye perceived all of the variations in color by combining these internally. When the both red and blue light but no green light enters your eye, you "see" magenta even though the light is not magenta. A combination of red and green, gives the perception of yellow while our eyes turn blue and green light into cyan.

Young's work had the form of a hypothesis. It remained for the physiological psychologist Hermann von Helmholtz a century later to postulate the existence of three types of color receptors, called cones, in the human eye that are stimulated by broad regions of the visible spectrum. Red light in one of these regions stimulated one type of cone, green light from the middle region could stimulate a second type of cone, and blue light in the final region stimulated the remaining cone. The relative degree of stimulation of these cones gives us perception of all of the colors that we see. We perceive sunlight as "white" because radiate from each of the three of the parts of the visible spectrum (red, green, and blue) stimulate the three cones in our eyes. If an object reflects red and green light but not blue light, my eyes will see it as yellow. If a second object reflects just red and blue light, my eyes will see it as magenta. Yet another object reflecting just blue and green light appears cyan colored.


We can observe how this works if we look closely at a computer monitor or a color television. The inside of the screen is coated with dots of chemical compounds called phosphors that will emit red, green, and blue light when they are stimulated by electrons fired in focused beams from three separate "guns" at the back of the tube. When you see a patch of yellow on the computer screen, the red and green phosphors at a cluster of these dots are excited and emit light while the blue phosphor in the cluster remains unstimulated. We see the combination of red and green light as yellow light. If you use a magnifying glass to look at the colored regions below, you will see the individual phosphors lit up. NOTE AGAIN THAT FROM A DISTANCE THE INDIVIDUAL POINTS OF LIGHT BLEND TOGETHER -- THE LIGHTS ARE MIXED IN THE SAME ADDITIVE WAY THAT NEWTON MIXED HIS COLORED LIGHT.

If you've been reading this skeptically, a good habit in reading, you may be wondering how this can possibly make sense in light of what you learned about colors in kindergarten. At an early age, most of us did a lot of coloring with crayons. We were taught, and soon verified for ourselves, that the primary colors were red, blue, and yellow. Using our crayons, we could combine any two of these to make the secondary colors: blue and red made purple, blue and yellow made green, and red and yellow made orange. The color wheel that we could make with crayons is quite different than Newton's color wheel. The primary colors that we learned in school, were red, blue, and yellow (NOT GREEN).
subtractive color wheel This wheel was made in a similar way to that which we are going to construct our egg tempera color wheels: I used only three pigments. These, when used alone, produced the "primary" colors:   red (top), blue (lower left), and yellow (lower right). Each of the "secondary colors", green, orange, and purple, were produced by coloring the same wedge with two different crayons. If you look at the wheel critically you'll notice that something is a bit off: the purple wedge appears very "muddy" -- not a nice purple at all -- and the orange wedge is slightly "muddy" as well. (We are going to spend some time considering why some pigments work well together and why others don't. By the end of the class you should be able to explain this to just about anyone.)


The idea embodied in our grade-school color wheel is important. Artists have known for a long time that they could prepare a paint of a third color by mixing paints of two pigments. This is useful only if the results are predictable. And our color wheel does just that. It predicts that mixing blue and yellow gives green. In this particular case, it also tells us that the blue and red crayons in my box don't give an especially vibrant purple. Through experience, painters develop a keen sense of which pigments mix well to produce certain colors, which will give muddy colors or bright colors. In our class project, I want you to get some of the experience of being the painter who uses his eyes to tell him which mixtures of pigments will give useful colors for making paint. I also want you to be the chemist, who can use a conceptual model to explain why some new colors in our wheels are muddy and others aren't. Ultimately, we would like to be able to predict the color that would result from mixing two pigments.

Let's consider for a moment the notion that the colors we see most of the time are the result of some sort of filters subtracting portions of the spectrum. If we had something that would take all of the RED out of WHITE LIGHT, the only two types of cones that would be stimulated in our eyes would be theose sensitive to the green and blue regions and we would see cyan. A cartoon of the three ideal subtractive filters that remove just the red or just the blue or just the green are shown at the right.filters

What would happen if, there were two filters in sequence?
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