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Here, a magenta filter blocks green light and a cyan filter blocks red light (see below; click to enlarge). Only blue light can pass through to your eyes. The banana looks blue because the white fruit reflects some blue light. But the yellow peel looks black. It reflects no blue light.

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An ideal red filter transmits only red light and absorbs all other colors. In this ideal case, a picture containing red, green, and blue would appear red and black when viewed through a red filter.

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Magazine illustrations are colored with pigments, but when you look at the illustration, you are sensing the light that is being reflected to your eyes.

A red filter blocks green light and blue light: Only red light can get through to your eyes. The white banana and the yellow peel both reflect some red light, so the whole banana looks red (see below; click to enlarge):

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A brightly colored picture takes on a whole new look when you view it through a colored filter, which transmits some colors and absorbs others. Using a colored filter, you can even decode secret messages written with colored pens or crayons.

Most colors of light can be made by mixing three primary colors of light: red light, blue light, and green light. Most colors of pigments, however, can be made by mixing magenta, cyan, and yellow pigments. Magenta pigment reflects red and blue light. More important, it absorbs, or subtracts, green light. Cyan pigment subtracts red light. Yellow pigment subtracts blue light. Four-color printing in magazines is done with inks of these three colors—magenta, cyan, and yellow—plus black. Mixed properly, these pigments produce the entire range of colors found in colored pictures in magazines.

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When you view multicolored writing through an ideal red filter, only red light reaches your eyes. Red light comes from both the red letters and the white paper (since white contains all colors). The red letters tend to disappear because they blend right in with the red light from the white paper. Letters that contain no red would appear black. Because most pigments are not perfectly pure, you may notice that more than just the red letters blend in with the background. That is, if a yellow letter reflects red (since yellow light can be made from a combination of red light and green light), the yellow letter would blend in with the background.

A yellow filter blocks blue light, so only red light and green light can get to your eyes. The white banana and the yellow peel both reflect some green light and some red light. The whole banana looks yellow because green light plus red light mix to make yellow light (see below; click to enlarge):

Likewise, a pure blue filter transmits only blue light, and a pure green filter transmits only green light. Any color from a picture that is not transmitted by the filter will be absorbed by the filter and will not be seen.

Place one colored filter at a time over a colored picture and notice how the colors are affected. With a red filter, the picture appears entirely in shades of red plus black.

Print your name or a short message on a piece of white paper using a different color of crayon, pencil, or pen for each letter. Then look at the message through a red filter. You may notice that the red letters disappear, but you can still see blue or green letters. By figuring out which colors you still see and which you don’t, you can write a secret message and then use the filter as the decoder.