Newton’s Color Wheels

Introduction

What is color? Why do we see different colors? In this demonstration, the concept of color will be demonstrated using a centrifuge device.

Concepts

• Reflection of light
• Primary colors
• Colors of light
• White light
• Persistence of vision

Materials

Safety Precautions

Make sure the paper disc is securely connected to the rotating platform so that it does not fly off during the demonstration. Allow the rotating platform to come to a complete stop before changing the disc. Do not touch the motor axle while rotor is spinning. Remove battery from Bracken’s Demonstration Spinner when not in use and during storage.

Disposal

All materials may be saved for future use.

Prelab Preparation

1. Obtain the color wheels sheet and a pair of scissors. Cut out the three color wheels.
2. Heat a paper clip or dissection needle with a candle or burner.
3. Melt holes in the centers of all of the color wheels and the given plastic disk.
4. The holes of each disk should fit snugly over the axle of Bracken’s Demonstration Spinner.

Procedure

1. Place the plastic disk onto Bracken’s Demonstration Spinner so that the motor axle extends through the middle of the disk.
2. Place the large green and blue color wheel onto Bracken’s Demonstration Spinner so that the motor axle extends through the disk.
3. Instruct students to look at the colored sections as they appear on the disk.
4. Start the motor and have students observe the resulting color of the spinning disk.
5. Turn off the motor.
6. Place the medium-sized green and red color wheel on top of the green and blue disk.
7. Start the motor and have students observe the resulting color of the spinning disks.
8. Turn off the motor.
9. Place the smallest wheel (the blue and red wheel) on top of the green and red disk.
10. Start the motor and have students observe the colors formed by all three spinning color wheels.
11. Turn off the motor. Repeat the demonstration, if necessary.

Teacher Tips

• Bracken’s Demonstration Spinner (Flinn Catalog No. AP6202) is required and sold separately.
• If the holes of the color wheels become worn, use a piece of tape to secure the wheels to the plastic disk or to the other wheels for added stability.
• Students may want to experiment making their own color wheels using different colored markers. Highlighters tend to work well.
• White light (or nearly white light) may be produced by spinning a disk sectioned in thirds and colored with green, pink and blue highlighting markers.
• If available, a document camera or alternative video projection system may allow the entire class to better view the spinning disc.
• Ask students to predict what colors will appear before performing the actual demonstration.

Discussion

Persistence of vision allows humans to combine the colors of the light as a disc is spinning. The white light from the classroom hits the surface of the spinning disc but reflects back to our eyes in different colors based on the color of the disc pattern.

White light is composed of light energy from the entire visible spectrum. Classically, the visible spectrum has been called “ROYGBIV” after the various colors of light.

ROYGBIV = Red, Orange, Yellow, Green, Blue, Indigo and Violet

Primary Colors of Light = Red, Green and Blue

The color of a substance results from the reflection of light from that substance. A red shirt appears red when exposed to “white” light because the red-colored light waves are reflected from the surface of the shirt and the other wavelengths are absorbed. However, most materials do not reflect a pure single-frequency color and absorb all the other frequencies. A “yellow” shirt can appear yellow due to the reflection of mostly “yellow” light frequencies or it can appear yellow because a mixture of light frequencies (red and green) reflect from a surface and are interpreted by the eye to be the color yellow.

A disc containing alternating blue and green colored sections will appear to be a single different color if the disc is rotated fast enough because the human eye cannot distinguish between the colors. Human eyes will perceive this as a cyan-colored disc as if they were being excited by simultaneous beams of blue and green light. Likewise, a spinning red and blue disc will appear magenta, and a spinning red and green disc will appear yellow.

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

An important point about the color of an object is that an object can only reflect the light frequencies that are present in the light that illuminates it. Since most objects do not completely absorb all the light frequencies (a red object may still reflect some “blue” and “green” light frequencies at a very low level), the color of the object will depend on the light source. A candle flame produces light that has more “yellow” frequencies than reds and blues so objects that are illuminated by a candle flame will appear more yellowish. An object that appears blue under “white” light will appear red under red light that has no “blue” frequencies. (An object will appear black if it does not reflect any red light.)

References

Flinn Scientific would like to thank Jeff Bracken, chemistry teacher at Westerville North High School in Westerville, Ohio for sharing this original idea. Jeff would like to thank Matt Cocuzzi, his student laboratory assistant, for his numerous creative suggestions during the development of this classroom demonstration. Gore, G. R. The Physics Teacher, 1982, 20, 101. Hewitt, Paul G. Conceptual Physics, 3rd ed.; Addison Wesley: Longman: CA, 1999; pp 422–424. Bartels, R. A. The Physics Teacher, 1986, 24, 564–565.