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Science in Focus: Shedding Light: Highlights

Workshop 3

primary colors | light bulbs | monochromatic pigments

Primary Colors

Primary colors are the set of colors that when combined in certain proportions will produce any color of the visible spectrum. These colors are perceived by the human eye, and come from light that may be:

  1. emitted from a source directly to the eye,
  2. reflected off an object, or
  3. transmitted through an object such as a filter

Primary colors may either be additive or subtractive. The additive primary colors are red, green, and blue. The subtractive primary colors are cyan, yellow, and magenta.

Explore the Resources for some hands-on explanations.

Additive Color Mixing

Photo 1

Additive Color Mixing of the primary colors of light demonstrates combinations which produce an array of lighter, brighter colors, including white. Mixing red and blue light, for example, produces magenta. Mixing blue and green creates cyan, and amazingly, red and green light combine to produce yellow.

A color television uses combinations of pixels emitting red, green, and blue light to form all the colors of the images on the screen. In a white region on the screen, all three types of pixels are illuminated.

Look at the example on the right of the three additive primary colors of light. Notice how white is produced at the intersection of all three.

Any two colors of light which produce white light when mixed are considered complementary.

  • Blue + Green = Cyan
  • Red + Blue = Magenta
  • Green + Red = Yellow
  • Blue + Green + Red = White

Subtractive Mixing of Primary Pigment Colors

Photo 2

The subtractive mixing of primary pigments produces an array of colors and even black. It's called subtractive because the pigment reflects some colors but abosorbs, or cancels out, others. A color printer uses tiny dots of magenta, cyan, and yellow to produce a multitude of colors and black. You can often observe dots of magenta, cyan, and yellow in color newspaper printing, as well as on the edges of ice cream cartons and other printed materials.

Mixing colors of pigment (paint, crayon, filters, e.g.) produces colors which are darker since more and more light is subtracted when they are combined.

Any two colors of pigment which produce black are considered to be complementary.

Resources

Light Bulbs

Have you noticed that light bulbs give off different kinds of light? Some are "warm" light, others "cool" light. What causes these differences?

White Light Bulbs
Normal incandescent (or filament) light bulbs contain a tungsten filament, selected because of its high melting point. As electrical current flows through the filament, it becomes 'white-hot' emitting heat (infrared) and visible light. The human eye perceives the light as white.

During manufacture of the incandescent light bulb, the air is pumped out of it. Argon (an inert gas) is pumped in to replace the air. This keeps the tungsten filament from 'burning' by reacting to elements in air (also known as oxidizing). This keeps the light bulb from catching on fire when it gets very hot.

A fluorescent bulb is a bit more complex. Theses light bulbs do not make use of filaments. Instead, they use electrodes at either end of the bulb. These electrodes pass energy back and forth within the bulb. As the energy flows within the tube, it hits the mercury vapor, which is inside of the tube. This causes the emission of ultraviolet energy. Ultraviolet energy is not visible to the human eye. However, the inside of the light bulbs are coated with substances known as phosphors. The phosphors glow when they absorb ultraviolet energy. This 'glowing' produces visible light.

Although the human eye perceives a white light from the fluorescent bulbs similar to that from an incandescent light bulbs, an analysis of the spectrum of energy given off from fluorescent light bulbs indicates that the light is more of a blue-white light. Fluorescent bulbs produce light more efficiently than incandescent (filament) bulbs; there is little energy wasted as heat.

Color Light Bulbs
One easy way of producing a colored light bulb, either incandescent or fluorescent, is to coat it with a translucent colored paint or film. If coated with red, for example, most of the colors from the emitted white light will be absorbed by the coating, but the red photons will be transmitted.

Another method for producing colored light is the use of gas discharge tubes. When the current flows, light is emitted at particular energy levels, characteristic of each individual element. For neon, the color is red to the human eye. For sodium, the light appears yellow. If we analyze each of the spectrums of the light, we can detect individual photon energy levels emitted; this provides a unique "fingerprint" for the light. Since each element has its own characteristic emission spectrum, materials can be analyzed precisely for their composition based on this phenomenon.

Resources
For more information on analyzing the spectrums of different kinds of light, check out these Web sites:

Monochromatic Pigments

Manufacturers are able to produce pigments which absorb one narrow energy range of photons and reflect all others. These are known as monochromatic (one color) and are quite expensive. Less expensive pigments are used in construction paper and children's paints. Consider the difference between the two types of pigments.

a) Suppose a box has been coated with high quality monochromatic green paint. If green light shines on this box it will reflect green. It will reflect green if white shines on it (and absorbs the other colors). But if red or blue shines on it, they are absorbed and it appears black.

b) Suppose the box is coated with a less expensive green paint which reflects a range of many colors, especially green. In this case, you would be able to detect the box if almost any color shone on it, but the color you perceive depends on the color of light shining on it.




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