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Project Summary

Difficulty	6
Time required	Very Short (a day or less)
Prerequisites	None
Material Availability	Specialty items
Cost	Average ($50 - $100)
Safety	No issues

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Abstract

Objective

The goal of this project is to create a colorful mini light show and shadow wall to learn about mixing and subtracting the primary colors of visible light.

Introduction

We're all familiar with the quirky dark shadows we can create with our hands using a bright light projected onto a wall or screen. In this project, you'll take dark shadows and transform them into amazing colors when you create a mini shadow wall that's anything but dark. This technicolor light show seems like magic and looks like a colorful work of glowing art, but it's also a cool demonstration of the physics and science of light and color.

Click here to watch a video of this investigation, produced by DragonflyTV and presented by pbskidsgo.org

The project video shows how five science buddies successfully met the challenge from their teacher to make a colorful art exhibit without using paint. The budding scientists put their heads together and developed several clever and colorful art displays each using light as the source of color. One of their more impressive "art pieces" was a multicolor shadow wall similar to a smaller version you'll set up in this project. The students found their initial inspiration for their shadow wall during a visit to The Exploratorium, an amazing science museum in San Francisco. There they learned that the trick to creating a wall of colored rather than dark shadows, is to shine multiple colored lights onto the screen instead of using standard "white" lights. That sounds fairly simple and logical.

But what surprised the science club when they constructed their shadow wall was the discovery of shadows made up of colors that were different from those of the colored lights they beamed against the screen. For example, if the lights shining on the screen were red, green and blue, some of the shadows that appeared were pink, turquoise, and yellow. Also, when they blended certain colors of light together they sometimes got unexpected results showing up on the screen. Red and blue lights together made the screen look magenta, a bright pink color, not purple like they were used to seeing when mixing paints.

You'll figure out why this happens and be able to predict which color will pop up in which shadows of your shadow wall after you study the results from a couple of simple experiments described in this activity. But first, it might be helpful to review a few basics about light and color.

Light is a form of electromagnetic energy that moves in wave-like patterns. We only know it's there because photoreceptors in the retina at the back of our eyes detect the incoming light waves and transfer that information to our brain via the optic nerve. In the brain, the signals are interpreted into information that tells us not only what we see, but also what colors are present before us. The color information originates from special photoreceptors in the retina. These conical-shaped mini-sensors are aptly called "cones." Each cone type is sensitive to a broad range of wavelengths of light, but each type has its peak sensitivity in a different part of the spectrum. We have long-wavelength cones that are most sensitive to red hues, short-wavelength cones that are most sensitive to blue hues, and mid-wavelength cones that are most sensitive to green hues. When all three types of cones are stimulated equally, the signals blend together in our brain and we see "white" light. If the cones are stimulated to different degrees, we see variations of color mixed from the basic red, green, or blue hues. Slight changes in relative signals from the three types of cones produce the millions of colors our eyes can detect.

Visible light usually looks colorless to our eyes, but is actually composed of a broad range of colorful light divided into bands by their wavelength, energy, and the color each produces when it hits our retina. These are the familiar rainbow colors of the visible light spectrum that separate from one another when visible light passes through a prism or through a rainbow's water droplets in the sky.

Figure 1. The visible spectrum includes wavelengths of light ranging from 400 to 700 nm in length (Illustration from Abrisa Glass & Coatings, 2005).

In this project, you will work with flashlights that produce the three "primary" colors of light— red, green, and blue— to see how they interact and blend together to yield multiple new colors on the background of the wall (see Figure 2, left panel). Primary colors are called primary because they can be mixed to produce just about every other color our eyes detect. In the most simple dual combinations of primary colored light waves, red and green mix together to make yellow light; green and blue mix together to make a turquoise-like color called cyan; and blue and red mix together to make the bright pink-purplish color called magenta.

Figure 2. Red, green, and blue light combine to make white light (left panel). Mixing two colors at a time produces yellow, magenta, or cyan colored light. That's very different from mixing colors of paint or inks (right panel). Note that combining three primary colors of inks produces black, not white.

You'll discover from your shadow wall how a color from one light can fill in what would normally be a dark or black shadow produced from another light shining on the same object. For example, when you hold your hand up in front of any color light, you block that light from hitting the wall and that creates a dark spot or shadow. But if another light, say green, is also beamed onto the wall from a slightly different angle, the green light spreads over the dark spot making it now appear green instead of dark. If that green shadow receives additional light beaming in from a third colored light source, the green light will mix with that color light to change the shadow into one of the other color combinations described above.

So in general, by looking at the placement and color of a shadow on a shadow wall, you can infer both the location and colors of the original light sources shining on the screen. You also can observe how the additive process of layering and blending a few colors of light can create a remarkable number of possible mixed color variations. Who knew studying shadows could shed so much light on our understanding of color!

To get started on this project, do some background research on light, vision, and color. You'll see a list of suggested search terms and a few basic questions to investigate in the next section. Then it's time to dim the lights, set up that shadow wall, and let the colorful show begin!

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Light
• Color
• Visible spectrum
• Light absorption
• Primary colors of light
• Colored shadows
• Cones, retina, and color vision

Questions

• What wavelengths of light make up the visible spectrum?
• How do the cones in the retinas of our eyes allow us to see color and so many colors?
• Why are shadows usually dark?
• How can you create shadows that have color?
• How do colors of light mix to produce new colors? What color combinations are possible using the primary colors of red, green, and blue light?

Bibliography

• For a picture of a colored shadow wall at the Exploratorium Museum in San Francisco, CA:
  Eppstein, D., 2001. "Colored Shadow Wall." [accessed June 5, 2007] http://www.ics.uci.edu/~eppstein/pix/sv/ex/ColorShadow.html.
• For background information and a similar experiment using colored shadows:
  Doherty, P. and Rathjen, D., 1995. "Colored Shadows," The Magic Wand and Other Bright Experiments on Light and Color (An Exploratorium Science Snackbook), Wiley. [accessed on June 5, 2007] http://www.exploratorium.edu/snacks/colored_shadows.html.
• An introductory site that explains color mixing:
  Truscio, R., 2005. "Introduction: Two Kinds of Color Mixing," Color Mixing website. [accessed June 15, 2007] http://home.att.net/~RTRUSCIO/COLORSYS.htm.
• More comprehensive site that explain how we see color and how color can be mixed:
  O'Haver, T., 2001. "The science and mathematics of color," In Living Color website. [accessed June 5, 2007] http://www.inform.umd.edu/UMS+State/UMD-Projects/MCTP/Courses/ColorLesson/.
• The visible spectrum illustration in the Introduction is from:
  Abrisa Glass & Coatings, 2005. "Understanding Light and Colort," Abrisa Glass & Coatings [accessed June 15, 2007] http://www.abrisa.com/guide/understandinglight/light.spectrum.asp.
• TPT, 2007. "Light and Color by Peter, Emma, Aaron, Ixchel, and Kaitlyn," DragonflyTV, Twin Cities Public Television. [accessed June 5, 2007] http://pbskids.org/dragonflytv/show/lightandcolor.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• A room where you can dim the lights
• White wall or piece of white paper taped to a wall
• A pen to serve as a "shadow maker"
• Small lump of clay to serve as an anchor for the pen
• Small table or stand
• Two small white light "Maglights" or pen flashlights of equal size and brightness
• Three LED micro-flashlights of equal size and brightness in red, green, and blue ("Photon Micro-Light II" key chain lights work well for this project because they have an on/off switch, are exceptionally bright, show true colors, are generally available, and are moderately priced. They can be purchased online at various sites, or check your local camping or electronics supply store. LED flashlights with similar properties from other manufacturers will do just as well, however.)
• One thick book to set the flashlights on
• Notebook
• Pen or pencil

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Experimental Procedure

1. These experiments can be done with the room lights on, but if you dim the lights even slightly, the colors of your mini shadow wall will be greatly enhanced.
2. To keep track of your observations, set up a data collection chart in your notebook similar to the example shown below. As you do each experiment, you can use the chart to record your observations in the appropriate boxes. The first row of each experiment in filled in already to show examples of the data you might collect. Any additional notes can be recorded below the chart.

Experiment 1: Dark Shadows (use the white light flashlights)

1. Set up the pen as your "shadow maker" on a table in front of a white wall. You can use the lump of clay to anchor the tip of the pen so that it stands upright on the table. Place the pen about 15 cm (approximately 6 inches) from the wall.
2. If the wall isn't white, tape a sheet of white paper to the wall behind the table and pen.
3. Place a thick book on the table about 30 cm (approximately 12 inches) from the wall. The book will serve as a support stand for the flashlights.
4. One Light Shadow Wall: Turn on one flashlight, set it on top of the book, and shine it directly toward the pen. Slide the light side to side. Observe and record the color of the shadow. Also note how the shadow position changes as you move the light.
5. Two Lights Shadow Wall: Turn on the second flashlight and place it next to the first flashlight. Shine both flashlights toward the pen, keeping the flashlights equal distances from the wall. Observe and record in your data chart what you see on the shadow wall as you slowly slide the lights side to side.
6. Hold both lights in your hands, switch off one light. Note the position of the shadow that remains. Observe what happens when you switch off the other light instead.

Experiment 2: Technicolor Shadows (use the colored Micro-Lights)

1. For these experiments, make sure the three flashlights are the same brand and brightness.
2. One Light Color Shadow Wall: Turn on one colored micro-light, place it on the book, and shine it toward the pen. Slide the light from side to side and observe any changes on the shadow wall.
3. Using your data chart, record the number of shadows, their color(s) and placement as you move the micro-light side to side. Also record the color of the background light appearing on the wall.
4. Repeat the process with the other two colored micro-lights, one at a time. In your data chart, record your observations about the number, color, and placement of shadows as you move each light from side to side. Also record the color of the background light for each micro-light.
5. Two Light Color Shadow Wall: Make observations of these three combinations of lights:
   • Red and green micro-lights
   • Green and blue micro-lights
   • Blue and red micro-lights

6. For each combination of lights, place the two micro-lights side by side on the book. Keep them the same distance from the wall as you shine their light toward the pen.
7. Slowly slide the lights apart on the book and then back together. Record the color, number, and position of shadows you see on the shadow wall when you move the lights. Also note the color(s) of the background.
8. Switch off one light at a time. Observe and record in your data chart the color and position of the remaining shadows(s). Also note any changes in background color.
9. Fill out all cells of the observation table for Experiment 2.
10. Three Light Color Shadow Wall: Place all three micro-lights on the book, the same distance from the screen.
11. Remove the pen from the table momentarily. Turn on two lights, and watch the screen while you turn on the third light. Observe and record the background screen color when all three lights are on.
12. Put the pen and holder back onto the table and move the pen up and back from the screen until you see three distinct shadows. Observe and record the color and position of the shadows.
13. Turn off one light. Observe and record the changes in color and position of the shadows along with any change in background color.
14. Repeat the process of turning off one light at a time with each of the other two micro-lights. Make a record of your observations on the wall each time.
15. Remove the pen and use your fingers as shadow makers. See how many different colors and patterns of shadows you can make with your fingers.

Analyzing Your Data

1. Looking at your observations from Experiment 1:
   1. What were the colors of the shadows from the white flashlights?
   2. How many shadows could you produce with one flashlight? with two flashlights?
   3. In the Two Light experiment, what happened to the shadows when you turned off the flashlight in your right hand? In your left hand?
2. Looking at your observations from Experiment 2:
   1. How were the shadows from a single colored light different from the shadows from two colored lights? Were these observations what you expected?
   2. What background color(s) appeared on the wall from each individual micro-light? What background color(s) appeared when you used two colored micro-lights? Were these colors the mixed colors you expected?
   3. What happened to the shadows and the background color on the wall when you turned off one of the two colored micro-lights? How did this result compare to what happened when you turned off the other micro-light instead?
   4. Under what conditions were the shadows from Experiment 2 similar to the shadows of Experiment 1?
   5. What was the color of the background wall when all three micro-lights were on?
   6. What color(s) were the shadows when all three micro-lights were on? Was this what you expected?
   7. For each of the shadows that you saw when the three micro-lights were on, determine which colors combined to make that color and which color was absent.
   8. What happened to the colors of the shadows when you turned off one of the three micro-lights? Was this what you expected?
   9. How did the shadows look when you used your fingers as shadow makers instead of the pen?
3. For a guide on how to summarize your results and write conclusions based on your data, see Conclusions.

Variations

• Try to make orange and violet colors: Set up the three micro-lights with the blue light in the middle. Shine the lights on the pen and see if you can adjust one micro-light at a time so that the yellow shadow becomes orange or that the magenta shadow becomes violet (purple). [Hints: Both orange and violet colors of light are made when there is an unequal, rather than equal, mix of two colors. To figure out how to create orange light, remember first that yellow light is an equal mix of red and green. When green is no longer equal to red, but is instead slightly less intense, the two colors still mix but they produce orange instead of yellow. Likewise, equal amounts of red and blue produce magenta light, but slightly less red will turn the magenta shadow into purple.]
• Create additional variations in colors: Use the three micro-lights as in Experiment 2, but place horizontal strips of red, blue, and green paper on the wall above a small strip of white paper. Observe the slight differences in colors that result when the shadows appear on the various background papers. Since each of the colored papers will be contributing more of their own color to the mixes of light you see on the wall, try to explain what relative changes in colors have occurred to produce the variety of hues on the wall.
• And let there be more light: Add a fourth color to the light sources. Purchase a yellow micro-light and repeat Experiment 2. Try to figure out what light combinations come together to produce the new shadow variations when you have four lights shining on the pen.
• Use color filters to subtract colors from the shadow wall: Purchase a set of colored filters or use pieces of clear colored plastics or cellophane to discover what happens to the shadow wall when you shine the micro-lights through various color filters in front of the pen. In these experiments, filters will reduce or eliminate all colors except the color they appear to our eyes. So a blue filter lets primarily blue light through but not the other wave lengths or colors of light. Try to explain the resulting colors in the shadows that appear on the wall.
• For another basic Science Buddies project idea describing how to mix colors of light as well as dyes, see "Mixing Light to Make Colors".
• For a more advanced project on mixing colors of light where you build an electronic circuit board to test and mix primary colored LEDs, see "Color Mixing with Red, Green, & Blue LEDs".
• To learn about mixing the colors of inks and dyes used in computer graphics and how it differs from mixing colors from lights, see the Science Buddies project, "Color Profiles".
• To better understand what color saturation and contrast mean in digital images, see the Science Buddies projects "Color Saturation" and "Digital Photo Contrast".

• For more science project ideas in this area of science, see Physics Project Ideas.

Credits

Darlene Jenkins, Ph.D.

Sources

The idea for this project came from this DragonflyTV podcast:

• TPT, 2007. "Light and Color by Peter, Emma, Aaron, Ixchel, and Kaitlyn," DragonflyTV, Twin Cities Public Television http://pbskids.org/dragonflytv/show/lightandcolor.html.

The experiments in this project were partially adapted from this activity:

• Doherty, P. and Rathjen, D., 1995. "Colored Shadows," The Magic Wand and Other Bright Experiments on Light and Color (An Exploratorium Science Snackbook), Wiley. http://www.exploratorium.edu/snacks/colored_shadows.html.

Last edit date: 2009-02-05 00:00:00

Career Focus

If you like this project, you might enjoy exploring careers in Physics.

	Physicist Physicists have a big goal in mind—to understand the nature of the entire universe and everything in it! To reach that goal, they observe and measure natural events seen on Earth and in the universe, and then develop theories, using mathematics, to explain why those phenomena occur. Physicists take on the challenge of explaining events that happen on the grandest scale imaginable to those that happen at the level of the smallest atomic particles. Their theories are then applied to human-scale projects to bring people new technologies, like computers, lasers, and fusion energy.			Nuclear Monitoring Technician Nuclear technology is used to image the human body, destroy cancer cells, sterilize food and medical equipment, create pest or drought-resistant seeds, and to generate power for 1 in 5 U.S. homes and businesses. Nuclear monitoring technicians help to keep the people who work with nuclear technology and the environment safe from excessive radiation exposure. They use special instruments to measure and monitor the radiation levels of workers, work areas and equipment, and they are involved in decontaminating work areas to safe levels. They also educate workers on radiation safety.
	Nuclear Medicine Technologist Many traditional medical imaging methods, like X-rays, can take pictures of certain parts inside the body, but sometimes these methods are not sensitive enough to detect a problem, or a picture is not enough—the doctor needs to see how a part is functioning, not just how it looks. That’s where nuclear medicine comes in. It can be used to see, for example, if bone repair is going on in a certain area, how a kidney is functioning, how a stomach is emptying, or how blood is flowing into and out of a heart. It can also be used to treat certain diseases. Nuclear medicine technologists are the special healthcare workers who administer radioactive drugs, take images of the patient, and then process, analyze, and show the computer images to the doctor.			Forensic Science Technician Guilty or not guilty? The fate of the accused in court lies with the evidence gathered at the crime scene. The job of the forensic science technician is to gather evidence and use scientific principles and techniques to make sense of it. It can be a grueling and graphic job, but very rewarding. If you like the idea of using science to help deliver justice, then you should investigate this career.

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