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Color

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Practice Color
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Color

Students will learn how our eye detects color and how color addition works. Reflection and transmission of color in various situations is also covered.

Key Equations

Red Green Blue Perceived color
$\checkmark$ $\checkmark$ $\checkmark$ white
black
$\checkmark$ $\checkmark$ magenta
$\checkmark$ $\checkmark$ yellow
$\checkmark$ $\checkmark$ cyan
Guidance
• White light consists of a mixture of all the visible colors: red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). Our perception of the color black is tied to the absence of light.
• Our eyes include color-sensitive and brightness-sensitive cells. The three different color-sensitive cells (cones) can have sensitivity in three colors: red, blue, and green. Our perception of other colors is made from the relative amounts of each color that the cones register from light reflected from the object we are looking at. Our brightness-sensitive cells (rods) work well in low light. This is why things look ‘black and white’ at night.
• The chemical bonds in pigments and dyes – like those in a colorful shirt – absorb light at frequencies that correspond to certain colors. When you shine white light on these pigments and dyes, some colors are absorbed and some colors are reflected. We only see the colors that objects reflect .
• Beautiful sunsets are another manifestation of Rayleigh scattering that occurs when light travels long distances through the atmosphere. The blue light and some green is scattered away, making the sun appear red.

Example 1

Question : In a dark room a blue light is shined on a magenta shirt what color does it appear? How about a yellow shirt?

Answer : Since magenta is the combination of red and blue and only blue light is available to be reflected, the shirt will appear blue. As for the yellow shirt, there is no blue light in yellow (yellow is the combination of red and green), thus the shirt will appear black since no light is reflected off it.

Time for Practice

1. Consult the color table for human perception under the ‘Key Concepts’ section and answer the questions which follow.
1. Your coat looks magenta in white light. What color does it appear in blue light? In green light?
2. Which secondary color would look black under a blue light bulb?
3. You look at a cyan-colored ribbon under white light. Which of the three primary colors is your eye not detecting?
2. The primary colors of light are red, green, and blue. This is due to the way our eyes see color and the chemical reactions in the cone cells of the retina. Mixing these colors will produce the secondary , or complementary colors: magenta, cyan and yellow, as well as white. On the diagram below label colors to show how this works:
3. Color printers use a different method of color mixing: color mixing by subtraction . What colors are used in this process? (HINT: look at the ink colors in an ink-jet printer, CYMB). The idea here is that each color subtracts from the reflected light – for example, cyan ink reflects blue and green but SUBTRACTS red. Explain with diagrams how CYM can be combined to produce red, blue, and green.
4. Answer the following light transmission questions
1. A beam of cyan light passes into a yellow filter. What color emerges?
2. A beam of yellow light passes into a magenta filter. What color emerges?
3. What color results when two beams of light, one cyan and one magenta, are made to overlap on a white screen?
4. White light passes through a cyan filter followed by a magenta filter. What color emerges?

Answers to Selected Problems

1. a. blue, black b. yellow c. red
2. .
3. .
4. a. green b. red c. blue d. blue