Mixing Colored Lights

Have you ever wondered why humans perceive the colors we see? By shining lights of three different colors against a white background, you better understand how human eyes “see” color.

Materials

• One red, one blue, and one green light. You can use three different colored light bulbs, each in its own socket; three different LED lamps, similar to those that a DJ uses; or three flashlights of equal size and brightness, each with a double layer of red, green, or blue cellophane attached with a rubber band.
• A white wall, paper, or poster board.

How To Do It

1. Tape a piece of white poster board or about 8 pieces of white copy paper to a wall in a dark room. (Notes: You can also use a white wall. A white projector screen does not work well.)
2. Turn the lights on and position them until you get the “whitest” light where the three colors mix. For best results, the room should be as dark as possible.
3. Place a solid object, such as a coffee cup, fairly close to the wall. Adjust the distance from the screen until you see three distinct colored shadows.
4. Turn off one of the colored lights, and notice how the color on the wall changes and notice the color of the shadows. Move the object close to the wall until the shadows overlap, and notice the color of these combined shadows.
5. Repeat the previous step with a different light turned off, while the other two remain on; and then a third time, until you have tried all combinations.
6. Repeat with just one color at a time, and then with all three. Can you make other hues by using your hand in front of the lights?

How It Works

When studying color perception, why do we use three lights of different colors? It’s because there are three kinds of color discriminating receptor cells, called cones cells, on the retina of the eye. The way we process the input from these three cone types produces a color circle of varying hues. In the 19th century, Ewald Hering first proposed that inputs from these three cone types are added and subtracted together to create three signals: brightness; redness vs. greenness (r/g); and yellowness vs. blueness (y/b). The 360-degree range of possible combinations of positive and negative r/g and y/b values creates the circular range of hues known as the color circle. Brain researchers later proved Hering’s theory to be true.

Colors on your monitor or TV screen are generated from phosphors or LEDs of just three colors, red, green and blue. Mixtures of light of these three colors create all of the colors that you see on the screen, including white and grey.

RED + GREEN = YELLOW

BLUE + RED = MAGENTA

GREEN + BLUE = CYAN

With red, green and blue lights, you can make shadows of seven different colors: blue, red, green, black, cyan, magenta, and yellow. If you block two of the three lights, you get a shadow of the third color. For example, block the red and green lights and you get a blue shadow. If you block all three lights, you get a black shadow.

When you turn off the red light, leaving the blue and green lights on, the lights mix and the wall appears to be cyan, a blue-green color. When you hold an object in front of this cyan color, you will see two shadows: one blue and one green In one place the object blocks the light coming from the green light and it leaves a blue shadow. In another spot it blocks the light from the blue bulb to make a green shadow

It may seem unusual that a red light and a green light mix to make a yellow light. A mixture of red and green light stimulates the red and green receptors on the retina of your eye. Those same receptors are also stimulated by yellow light! When the red and green receptors in your eye are stimulated, whether by a mixture of red and green light, or by yellow light alone, you see the color yellow!

For more information, please see articles on the theory of color and color pixels.

Variations

Use different colored paper on the wall. Try red, green, blue and yellow. Use neon colored paper to see a different effect. The brain’s ability to adjust to overall background color is called color constancy. This is true for muted colors, such as creams and beiges.

Use a pin to poke a small hole in an index card and hold it close to the wall. Vary the distance between the lights and the wall. You may see three colored dots and four different colored shadows.

There are other ways to make different colored lights. For example, you can buy colored photography filters (red, blue, green, cyan, magenta and yellow) and attach them to flashlights. You can also add food coloring to large glasses of water and produce red, green and blue lights by shining lights through the glasses of colored water. In addition, you can buy stage lights or DJ lights for RGB color combinations.