Combining Colored Light

Introduction

Finally, a simple way to combine the colors of light! White light is made up of all the colors of the visible spectrum. Most people have seen white light dispersed into its individual colors after passing through a prism. But most people have not seen different colors of light combined to make white light. This easy demonstration will delight your students!

Concepts

• Absorption
• Transmission
• Refraction
• Visible spectrum

Background

Isaac Newton (1642–1727) was the first to make documented studies of color and light. By passing sunlight through a prism, Newton showed scientists that white light was composed of a rainbow of colors. The colors spread out in a particular order—red, orange, yellow, green, blue and violet. Newton referred to this color pattern as a spectrum (see Figure 1).

Further studies have shown that various colors of light have different frequency ranges. When all of the visible light frequencies mix together, white light is produced. Surprisingly, white light can also be made from combining only red, green, and blue light of equal brightness. Figure 2 represents red, green and blue light projected onto a white screen in an overlapping pattern. White light is created in the center of the image where all three colors of light come together. This is possible because red, green, and blue are the primary colors of light—they can be used to create all other colors of light. When all three of the primary colors of light merge together, it is almost as if all the colors of the visible spectrum have come together (see Figure 2). This result should not be confused with the primary colors of pigments—red, yellow and blue. When the primary colors of pigments are mixed together, a brownish color results. Combining light and mixing pigments are very different processes.

This simple demonstration combines the three primary colors of light using simple materials. White light from the overhead projector passes through three colored transparent sheets—one red, one green, and one blue. The transparent material absorbs certain colors of the white light while allowing red, green, and blue light to be transmitted. With the help of convex lenses, these three colors will be refracted, or bent, and then overlapped with one another, creating white light.

Materials

Safety Precautions

This activity is considered to be nonhazardous. Follow all laboratory safety guidelines.

Disposal

The materials may be saved for future demonstrations.

Procedure

1. Obtain the red transparent acetate sheet and trim it so it will cover one of the holes on the white plastic sheet. The piece should be large enough to cover one hole, but should not overlap the other holes.
2. Place the red transparent acetate sheet over one hole and secure its edges to the plastic sheet using transparent tape (see Figure 3).
3. Cover the remaining holes by repeating steps 1 and 2 using the green and blue transparent acetate sheets, respectively (see Figure 4).
4. Turn on the overhead projector and allow students to observe the white light emitted.

5. Place the white plastic sheet on the overhead projector with the taped side down (see Figure 5). Red, green, and blue light will be projected as separate circles onto the white projection screen.
6. Focus the circular images on the overhead screen. The three colors should still be projected as separate circles.
7. Place a double convex lens over each transparent color circle on the overhead projector.
8. Slowly roll the overhead projector toward the overhead screen. The colors will start to blur and overlap each other. Stop moving the projector once all three colors of light have merged together in the center to form white light. Yellow, magenta and cyan light will form where two colors have merged together (see Figure 2 in Background).

Student Worksheet PDF

12700_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform this demonstration repeatedly. Once the overhead sheet is made, it can be used year after year. Store in a mailing envelope to reduce scratching.
• Use this demonstration to “hook” your students at the start of a lesson on color and light. This demonstration can be used to teach the concepts of absorption, transmission, and refraction of light.
• For future concept development try combining red, blue, and green colored paint in a clear container. Compare the outcome of this demonstration to the outcome of combining different color picments. Discuss the differences between light and pigment.
• For further concept development on this topic, the following kits are available from Flinn Scientific—the “Color and Light” Spectrum Demonstration Set (Catalog No. AP6172), “Newton’s Color Wheels” (Catalog No. AP6162), “Kaleidoscoptical Activity” (Catalog No. AP8781), and “Modeling Eye Optics” Student Laboratory Kit (Catalog No. AP6694).

Answers to Questions

Observations

1. Describe the color of the light emitted from the overhead projector.

The color of light emitted from the overhead projector is white.

2. Describe the initial colors observed in each circle when the plastic sheet is placed on the projector.

The initial colors observed are red, green and blue.

3. What is placed over each transparent color in order to refract the light and to combine the colors?

Lenses are placed over each transparent color in order to refract the light to combine the colors.

4. Record the name of the color produced when the following colors of light are combined.

Red and Blue = Magenta
Red and Green = Yellow
Blue and Green = Cyan
Red, Blue and Green = White

Post-Lab Questions (Student answers may vary.)

1. Using a textbook or another source of reference, define the following terms.

a. Visible light

Visible light is an electromagnetic wave that is seen by the human eye. Light is made up of elementary particles called photons which have no mass.

b. Transmission 

Transmission—When light is able to pass from one medium into another medium. 

c. Absorption 

Absorption—When light is retained in a medium and not allowed to pass into another. 

d. Refraction

Refraction—The bending of light when it enters a new medium. 

e. Pigment 

A pigment is a material that absorbs colored light.

2. When white light is passed through a prism, what is commonly observed?

When white light is passed through a prism, the visible spectrum of colors is observed.

3. Explain how the colored transparent sheets are able to change white light into colored light. Explain your answer using an example.

When white light passes through a colored transparent material, certain colors of light are absorbed and certain colors are transmitted. For example, the red transparent sheet absorbs certain colors of the spectrum while allowing others to pass through. Red is seen because of the combination of colors transmitted to the eye.

4. Explain how the lenses in this demonstration are used to combine the light from each circle.

The lenses refract or bend the light in order to combine the colors.

5. Do you think that mixing combinations of red, blue, and green pigments (paints) together will produce the same effect as mixing light? Justify your answer with an example.

No, mixing pigments together will not produce the same result as mixing light. Light and pigment are two different substances. If red, blue and green paint are mixed together, a color close to black will result, not white as in combining light.

6. Design an experiment in which the effects of combining colors of paint are compared to combining colors of light. Be sure the experiment includes the following.

a. The colors of paint(s) needed. 

In order to compare the effects of combining paint to combining light, red, blue and green paint would be needed. 

b. Other additional materials.

Additional materials needed for this experiment are containers for paint mixing and stirring instruments. 

c. The color combinations that must be tested.

The color combinations that must be test are as follows. 

Red and blue paint
Red and green paint
Blue and green paint
Red, blue and green paint