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Color Addition

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Author: Marc Rosner

The three pure primary colors of light are red, green, and blue. When two primary colors of light mix, a secondary color results.

  1. Red Light + Green Light → Yellow Light
  2. Red Light + Blue Light → Magenta Light
  3. Blue Light + Green Light → Cyan Light
  4. White light is a mixture of all three primary colors:
  5. Blue Light + Green Light + Red Light → White Light

If you're scratching your head and saying, "But red and green paint make brown!" it's because paint colors combine by color subtraction, not color addition. The pigments in paint filter (block out) reflected light. In pigments the primary colors are red, yellow, and blue. Color addition is the process of blending differently colored lights together, such as shining different colored theater lights onto a stage. The more colored lights you add to the blend, the brighter and whiter the results. Color subtraction is the process of successively filtering out colors. If you look through several pairs of different colored sunglasses, you can observe color subtraction. Similarly, the more colors of paint or dye you mix, the more you subtract and the darker and blacker the results.

Materials

  • 3 focused light sources (slide projectors or theater lights)
  • white screen
  • tape
  • colored gels or filters-red, blue, and green
  • Why is the sky blue? The atmosphere refracts (bends and changes the velocity of) light into different colors as a prism does. Blue is refracted the most, so it bends back to our eyes. For the same reason, water in the ocean reflects light rays, bouncing them off the surface, so the ocean often appears blue. (For more on refraction and reflecting, see the next chapter.)
  • prism
  • scissors
  • cardboard
  • drawing compass
  • metric ruler
  • cordless drill with sanding disk or buffing wheel bit (requires adult help)
  • glue
  • safety goggles
  • white paper
  • markers or paint and paintbrush

Procedure

To Demonstrate Color Addition with Gels

  1. Aim your three light sources at a white screen in a dark room. They should overlap, but not completely (see figure).
  2. Tape gels or filters in front of each light to make the three primary colors of light (red, blue, green) fall on the screen.
  3. The three secondary colors of light (cyan, yellow, and magenta) will appear where two primary colors overlap. At the intersection of all three colors, you will see white. (You may have to experiment with different shades of gels, or double up some colors, to get ideal results.)

Color Addition

Isaac Newton split a narrow beam of white light into the colors of the spectrum-red, orange, yellow, green, blue, indigo, violet-using a prism in a dark room. You can, too.

To Demonstrate Color Addition with a Color Wheel

  1. Cut out a circular cardboard disk, 15 to 20 cm in diameter. Devise a method of mounting the disk on a drill so that the disk spins. One way to do this is to have an adult center the disk on a sanding disk or buffing wheel bit and fasten it securely with glue. Use caution when you operate the drill-wear goggles and don't touch the spinning disk.
  2. Cut out a white paper disk of the same size, and use markers or paint to color it red, green, and blue in pie slices. Glue it to the cardboard wheel.
  3. Color Addition

  4. Spin the color wheel as fast as you can and observe the resulting color.
Since your colors are not likely to be pure and the same intensity, you will probably have to vary the size of the pie slices to get white-which may be closer to cream in color. In both activities, you can make a table of primary, secondary, and more complex (tertiary) colors as shown. (Fill in the blanks based on your experimentation.)

References

The Magic Wand and Other Bright Experiments on Light and Color (Exploratorium Science Snackbook series) by Paul Doherty (New York: John Wiley & Sons, 1991).

The Optics Book: Fun Experiments with Light, Vision, and Color by Sharon Levine and Leslie Johnstone (New York: Sterling. 1998).

Make a Splash with Color: www.TheTech.org/exhibits_events/online/color /intro/

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