We know white light is the presence of all colors. We see sunlight as white and we have discussed the experiments of Sir Isaac Newton in which he recombined the spectrum of all colors to produce white light. Our eyes and brains can be fooled somewhat. Figure 17.20 shows three spotlights, of red, blue, and green light, shining onto a screen in an overlapping pattern. These three colors are called primary colors, or additive primary colors. They correspond to the three kinds of color-sensitive cones in our retinas. In the region where both red and blue light strike the screen our eyes and brains see a rosy color we call magenta. In the region containing only blue and green light we see a blue-green turquoise color known as cyan. Those two do not seem unusual. But in the region where only red and green light are found, our eyes and brains see yellow! Perhaps even more unusual, in the central region where all three, and only those three, colors are found, our eyes and brains see white light! We see or distinguish or experience the same thing as if real white light, with all wavelengths, were there, even though only light of three wavelengths is present.
Figure 17.20 Three spotlights of red, green, and blue shine upon a screen in an overlapping pattern. Various colors are "seen" where these three colors overlap.
This characteristic of human vision is very useful in photography, printing, art, and television. As sketched in Figure 17.21, a color television screen or a color computer monitor is made up of thousands of tiny dots, called phosphors, which glow red, green, or blue when excited by an electron beam. All the colors that we see on a television screen are made of differing intensities and combinations of red, green, and blue.
Figure 17.21 A color television picture is composed of many, tiny red, blue, and green dots or "phosphors" that glow when excited by electrons.
Think of a desert scene with a puffy white cloud in a blue sky above a yellow sand dune beside which stands a green cactus with a red bloom, as sketched in Figure 17.22. A very close inspection of the television screen in the area of the blue sky will show that only the blue phosphors are lit. In the area where we see the green cactus, only the green phosphors are turned "on". In the area of the red cactus flower, only the red phosphors are "on". In the yellow sand dune, both the green and red phosphors are illuminated. In the white of the cloud, all three phosphors will be on.
Figure 17.22 Turning on or lighting up different sets of the red, blue, and green phosphors on a TV screen provides all the different colors you see.
Figure 17.E A color image like this on a television screen or computer monitor is composed of the three images, as seen here, in red, blue, and green. Remember, this sort of image is the result of color addition.
Q: In a television picture of a turquoise ring, what color phosphors will be lit?
A: Turquoise or cyan is made up of blue and green.
Q: Which phosphors will be lit in a television scene of Linus and the Great Pumpkin which is quite orange?
A: Lighting the red and green phosphors equally will produce yellow. If more of the red phosphors are lit than the green ones, or if the red phosphors are lit brighter than the green ones, the resulting color will be orange.
