White light goes through a filter that can absorb blue light what color of light can pass through

When red, green, and blue spotlights are "mixed" together, they produce white light. But you already know that mixing red, green, and blue paint together does not produce white, it produces a murky grey or muddy brown. What is the difference? When we mix paint, or ink or crayons or chalk, we are mixing pigments. White paint reflects all colors. To make the paint red, we mix in red pigment, small particles that reflect red light and absorb other colors of light as illustrated in Figure 17.23.

Figure 17.23 Pigment gives color to paint. Red pigment, illustrated here, is tiny particles that reflect red light and absorb other colors of light.


If we mix red and green pigment in white paint, the red pigment will absorb the green and blue light while the green pigment will absorb the red and blue light. The end result of this will be that no light is reflected at all, as illustrated in Figure 17.24. In actual practice, most pigments will not be totally absorbing for any color so the result will be our usual murky grey or muddy brown instead of actually being black. Pigment reflects what it does not absorb but its active role is absorbing. When paints are mixed together the result is the sum of their absorption characteristics. That is why this is known as color subtraction.




Figure 17.24 Mixing red and green paint together should produce black. In actual practice, it will probably produce a dark grey or brown.


We will also get black, or dark grey or brown, if we mix red (which absorbs green and blue) with blue (which absorbs red and green) or if we mix green (which absorbs red and blue) with blue. These are illustrated in Figure 17.25.





Figure 17.25 Mixing red and blue paint together or mixing green and blue paint together should produce black. In actual practice, these will probably also produce a dark grey or brown.


Cyan is light with a wavelength between green and blue or a combination of green and blue light. Cyan pigment will absorb the red and reflect green and blue or cyan colored light. Magenta is a combination of red and blue, without any green. And yellow is seen when red and green light are combined. These colors are illustrated in Figure 17.26.




Figure 17.26 Cyan results from absorbing red light from white light. Yellow results from absorbing blue from white. Magenta is the result of absorbing green out of white.


We have already seen what happens when we mix various combinations of red, green, and blue paint. Now we shall do the same with cyan, yellow, and magenta. Figure 17.27 shows cyan and yellow paint mixed together. The cyan pigment absorbs red while the yellow pigment absorbs blue. Only the green is reflected. It is not uncommon for people to refer to cyan as "blue" so you may already know this in terms of "yellow and blue make green".


Figure 17.27 Yellow and cyan paint mixed together produce green. Cyan may be commonly referred to as "blue" so you may already know this as "yellow and blue paint mixed together produce green".


Figure 17.28 shows the mixing of cyan paint and magenta paint. Cyan absorbs red while magenta absorbs green. Only the blue remains. Remember, when mixing pigments, it is the colors that are absorbed or taken out or subtracted that compound or accumulate.


Figure 17.28 Magenta paint and cyan paint, when mixed together, produce blue.


Filters placed in front of each other behave in the same way as mixing paint. Pigments in filters selectively absorb different colors and the remaining colors pass through the filters just as the remaining colors are reflected from paint. What happens if a yellow filter and a magenta filter are used together? The yellow pigment in the yellow filter absorbs blue light and allows the red and green light to pass through, making yellow. The magenta pigment in the magenta filter absorbs green and allows red and blue light to pass, making magenta. This is illustrated in Figure 17.29.





Figure 17.29 Overlapping magenta and yellow filters absorb all the blue and green light and allow only red light to pass through.


We began our discussion of color addition by shining three colored spotlights in an overlapping pattern. Figure 17.30 shows a similar overlapping pattern of colored filters. Overlapping filters of the subtractive primary colors, cyan, yellow, and magenta, allow the additive primary colors, red, green, and blue, to pass through where two of the filters overlap. Where all three filters overlap, no light passes through and the area is black.


Figure 17.30 Overlapping filters selectively absorb light of different colors and allow the remaining light to pass through. Overlapping filters of the subtractive primary colors, cyan, yellow, and magenta, allow the additive primary colors, red, green, and blue, to pass through where two of the filters overlap. Where all three filters overlap, no light passes through and the area is black.


Q: What color results when white light shines through a cyan filter and then a yellow filter?

A: The cyan filter absorbs red and allows green and blue light to pass on through (cyan is blue-green). The yellow filter absorbs blue. Since the light has only blue and green light in it after passing through the cyan filter, the yellow filter will absorb the blue and let only green light pass through.

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All the colors you see in this book, or any other book, can be made up of tiny dots of these three subtractive primary colors, cyan, yellow, and magenta. However, the blacks probably wouldn't look really black. They would just be the muddy grey or grungy brown that you get when you mix paints together. To take care of this, books are printed with a fourth ink, black. Therefore, full color books are often referred to as four-color printing. Printers and photographers sometimes refer to this full, four-color process as a CYMK process with C, Y, M standing for Cyan, Yellow, Magenta and K standing for black. Television and computer people often refer to an RGB signal or an RGB image with R, G, B standing for Red, Green, and Blue.

Different inks may produce slightly different colors. Acme's magenta ink may not be exactly like Widget's magenta ink. And the cyan ink from Wonderful Supply company may produce a more greenish color than the cyan ink from Wow'em Arts which seem a little more bluish. The green phosphors on an Apple monitor may be a little brighter than the green phosphors on a Radius display. The red phosphors on a Sony Trinitron picture tube may be a little different than the red phosphors on a Magnavox picture tube. There are adjustments that let you adjust the color on your television to suit your own tastes. Likewise adjustments on printing processes let a printer take into account variations in ink or printing presses or desktop printers.

Cyan, yellow, and magenta? Have you heard those colors before, cyan and magenta? You may not have. In grade school art class you may have talked about the primary colors as blue, yellow, and red. You can use those to mix colors very well, just as inks from different companies, with different color characteristics, can still be used in printing a full, four-color picture. However, your art teacher was probably thinking of cyan and calling it blue and thinking of magenta and calling it red just to make art and color mixing seem a little easier.

In specifying exactly what some color is to be, you could specify the amount of red, green, and blue light that it should contain. Some color controls on desktop computers let you pick a color that way. Some examples are shown in Figure 17.31.


Figure 17.31 Colors may be specified by giving the amount of red, green, and blue the colors should contain.


More often, however, colors are specified in terms of hue, saturation, and lightness (h, s, l); or these may be called hue, saturation, and brightness (h, s, b). Adjustments to your color television use this method. Some color controls on desktop computers also let you choose colors using this technique or process or terminology. Hue refers to the basic color; are we talking about a red or an orange or a red-orange? In terms of Figure 17.32, hue give the angular location on the color wheel there. You may think of saturation describing how much pigment you have added to white paint. With a high saturation, we have lots of red pigment to make a brilliant crimson red. With little saturation we have only a little red pigment to make a soft pink. Lightness describes how light or bright the whole color is. Even brilliant crimson red will look dark and rusty when illuminated by a candle in an otherwise dark room. Some additional examples are shown in Figure 17.33.


Figure 17.32 Hue, saturation, and lightness can be used to specify the characteristics of a color.


Figure 17.33 Colors may be specified by giving the values of the hue, saturation, and lightness the colors should have

Q: What color results when white light passes through a red filter and a magenta filter?

A: A red filter absorbs green and blue from the white light, leaving only red. A magenta filter absorbs only green and there is no green in the light by the time it reaches the magenta filter so this filter has no effect on it and the red light passes through unaffected.

Q: What color results when white light passes through a red filter and a yellow filter?

A: A red filter absorbs green and blue from the white light, leaving only red. A yellow filter absorbs blue light and lets red (and green) light pass through. Therefore, red light emerges from both filters.

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Visible light is electromagnetic (EM) radiation with wavelengths from about 400 nm (400 x 10-9 m) for violet light through about 700 nm (700 x 10-9 m) for red light. Our eyes are most sensitive to light in the middle of this range, such as yellow light with a wavelength of about 550 nm (550 x 10-9 m). Electromagnetic radiation with longer wavelengths is called infra-red (IR) radiation. Our eyes can not detect this IR radiation but our skin feels it as heat. EM radiation with wavelengths shorter than the visible range is called ultra-violet (UV) radiation. Again, our eyes can not detect this UV radiation.

When white light passes through a prism its direction is changed and it is spread out into various colors. This spreading of the light into various colors is called a spectrum. This spreading of colors in the white light is because the index of refraction varies with the wavelength (or color) of the light. The variation of index of refraction with wavelength is called dispersion.

Rainbows are caused by the dispersion of light as it is refracted and reflected by tiny droplets of water. The angle between the center of a rainbow and the rainbow itself is about 45o. The colors of the spectrum are present in a rainbow with violet and blue on the inside and red on the outside. A second rainbow is formed by light that is reflected twice inside the rain droplets. The colors on a second rainbow are reversed.

Rods and cones on the retina of our eyes allow us to see. The cones are color-sensitive and require higher levels of illumination. The rods are far less color-sensitive and come into play when the light is dim.

Color addition applies for spotlights or color television. The additive primary colors are red, green, and blue.

Color subtraction applies for paints or color printing. The subtractive primary colors are cyan, yellow, and magenta.