Color and Light

Introduction

Why is a red apple red? Does a green filter change white light into green light? What color of light is observed when red and green lights are mixed? The answers to these questions may surprise your students. This demonstration uses a holographic diffraction grating and an overhead projector to produce a very large, sharp rainbow. The large spectrum allows you to demonstrate the true nature of color and light and address common misconceptions related to the perception of color.

Concepts

• Visible light spectrum
• Color
• White light
• Reflection and absorbance

Materials

Safety Precautions

Although this activity is considered nonhazardous, please follow normal classroom safety guidelines.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The green food dye solution may be poured down the drain according to Flinn Suggested Disposal Method 26b.

Prelab Preparation

1. Obtain an overhead projector and place it 10–15 feet from a projection screen (or blank, white wall).
2. Turn on the overhead projector and aim the light at the desired screen or wall. Turn off the lights in the room and eliminate other extraneous light by closing the blinds or curtains.
3. Obtain the two black sheets of construction paper.
4. Form a 2-cm wide slit in the center of the stage of the overhead projector with the two sheets of black construction paper. Position the slit on the stage so that the image of the slit projected onto the projection screen is vertical (refer to Figures 1 and 2).
   {11992_Preparation_Figure_1_Overhead projector}
   {11992_Preparation_Figure_2_Projected spectra}
5. Place the holographic diffraction grating film above the lens of the overhead projector (see Figure 1). Be sure to wear gloves when handling the diffraction grating. If the spectra are not projected to the left and right of the screen, rotate the diffraction grating 90 degrees (the alignment of the grating is important). Once two bright spectra (mirror images of each other) are displayed horizontally to the left and right of the film screen (see Figure 2), the diffraction grating should be secured to the lens with tape. Tape only on the outside edges of the diffraction grating and make sure the diffraction grating is taped flat.
6. Adjust the focus of the overhead projector so that the image of the slit is in sharp focus on the screen. The two spectra should also come into focus.
7. Only one of the two spectra will be used during the demonstration. If necessary, adjust the position of the “selected” spectrum on the wall closer to the screen. To do this, move the construction-paper slit on the stage of the overhead projector to the left or right of the center of the stage. Make sure the projected image of the slit remains visible on the desired screen or wall (it will be needed as a reference image during the demonstration). Generally, positioning the slit on the stage so that it is half way between the center of the stage and the edge of the stage will work well to give one sharp, bright spectrum (the other spectrum will be farther to the side and out of focus). If your projector has an adjustable reflecting mirror this can also be used to adjust the position of the spectrum. Adjusting the width of the slit adjusts the width of the projected spectrum. Adjust as necessary to project a sharp rainbow spectrum with red, green and blue clearly visible. A 2-cm or slightly smaller slit generally produces a large, sharp spectrum.
8. Once the slit is aligned so the “selected” spectrum is in the proper location to be viewed by the students, the construction paper should be secured to the overhead projector with tape.

Procedure

Activity A: Why is a red apple red?
The color of an object is determined by the color that illuminates it, and the colors that the object reflects and absorbs.

1. Obtain the three colored construction paper sheets (red, blue and green).
2. One at a time, slowly move the colored paper across the “chosen” spectrum projected on the wall or screen.
3. Have students observe the appearance of the colored paper in the different colors of light. The “red” paper appears lackluster blue (or nearly black) when illuminated by blue light, and dull green when illuminated by green light. In the red region, the paper appears very bright red. Similar color displays occur for the other sheets of colored paper. The “green” paper appears faded red in the red light and dull blue in the blue light, and a very bright green in the green light. The “blue” paper appears dark red (nearly black) in red light and dull green in the green light, and bright blue in the blue light.
4. On a white sheet of notebook paper, make a large swirl pattern or design with a yellow highlighter marker that is large enough to be seen by the students in the class.
5. Slide the paper, with the swirl pattern facing the class, through the projected spectrum. Move from the red region to the blue region of the spectrum. The yellow design on the paper will be invisible to the students (or it may be very faint) in the red and green regions. As the paper moves into the purple and violet region, the yellow becomes bright and visible. Continue to move the paper out of the visible spectrum and it will continue to glow. This occurs because the highlighter pen’s ink is fluorescent. It turns violet and invisible ultraviolet light into visible light! This demonstrates to your students that white light is composed of more than just the visible spectrum.

Activity B. Does a filter change one color of light into another color?
When light passes through a filter, the filter absorbs certain colors and transmits and reflects other colors.

1. Obtain the flat-sided bottle and a black marker.
2. With the black marker, write a word on one of the large, flat sides of the bottle (e.g., spectrum, green).
3. Fill the bottle full with water and add enough green food dye (20–30 drops) to make the liquid dark enough to obscure the written word on the side of the bottle.
4. Move the bottle slowly through the projected spectrum on the wall, with the word facing the students. Make sure students are seated or standing where they can see the shadow cast by the bottle.
5. Observe the appearance of the liquid in the bottle and the projected shadow of the bottle on the wall. In red light, the liquid inside the bottle appears black and it casts a black shadow onto the screen. As the bottle moves into the green region, the liquid inside the bottle becomes transparent and does not cast a shadow on the screen. However, the word on the side of the bottle is visible and the word’s shadow projects onto the screen. As the bottle moves into the blue region the liquid becomes dark again and a shadow of the bottle is cast. The green dye reflects and transmits green light and absorbs the other colors.
6. In a step-wise fashion, demonstrate the effect of a filter by slowly adding a green food dye solution drop-wise into a Petri dish filled with water.
7. To do this, fill the Petri dish with enough tap water to just cover the bottom of the dish completely.
8. Obtain a 50-mL beaker and fill it with about 40 mL of tap water.
9. Add 4 to 5 drops of green food dye from the dropping bottle to the 40 mL of tap water in the 50-mL beaker and mix the solution.
10. Place the Petri dish with water on the overhead projector stage centered over the 2-cm wide slit. Make sure the liquid inside the Petri dish is illuminated and the image of the Petri dish and liquid in the slit is clearly visible on the projection screen. The spectrum should be visible as well. If necessary, allow the water to sit for a minute until it stops swirling. Refer to Figure 3.
    {11992_Procedure_Figure_3}
11. With the pipet, gently add one drop of the diluted green food dye solution to the center of the water in the Petri dish.
12. Observe the projected image of the green swirls in the liquid. How is the spectrum affected? Notice that in the region of the spectrum correlated to the green swirls in the slit, the areas of the spectrum that should be red and blue are now black, whereas the green and yellow sectors remain bright. Refer to Figure 3.
13. Continue to add the green food dye solution drop-wise and observe the effect the green dye has on the spectrum. The green color in the Petri dish becomes more distinct and dark, and the outer edges of the red and blue regions of the spectrum related to the Petri dish become black.
14. Add enough diluted green food dye solution to the Petri dish until the solution in the Petri dish is dark green (about 4 to 5-mL). Notice that the Petri dish region of the projected spectrum only displays green. The red and blue areas are completely black. Light not shining through the “filter” still displays a continuous spectrum. The red and blue colors have been absorbed (“filtered” out) by the dye and only the green color is transmitted. This demonstrates why the green liquid in the bottle casts a shadow in the red and blue areas of the spectrum. Red and blue light are absorbed by the dye. Green light is reflected and transmitted giving the dye a “green” color.

Activity C. What is the observed color of the light when red and green light are blended?
Red and green light mix to display yellow light.

1. Select three students to hold the three mirrors in the colored spectrum. Select a nearby wall that the light can be reflected to with the mirrors and still maintain a bright intensity. Tape the white sheet of construction paper onto this wall. Students will reflect the light onto this white target to display the reflected colors.
2. Students should position each mirror so that the shadow of the mirror is seen clearly in one of the red, green or blue areas of the spectrum.
3. Have the students with the “red” and “green” mirrors reflect the light onto the white paper target so that the two colors overlap. What color is seen? The red and green will mix and the reflected light appears yellow!
4. Then have the student direct the reflection from the “blue” mirror onto overlapping red and green reflections. What color is observed? White light is displayed! (i.e., no color).
5. Have the students experiment by overlapping other colors or by varying the intensity of the reflected light to form more colors of light red and blue produces magenta, blue and green produces cyan, etc. What happens when yellow and blue are mixed? “White” light is observed because yellow and blue are complementary colors, see Discussion. Students may predict that green light will be observed because they have seen yellow and blue paint combine to produce green-colored paint.

Teacher Tips

• The flat bottle filled with green food dye solution should be made once and saved for future demonstrations. The shelf life of this dilute food dye solution is approximately two months. It is vegetable based and will decompose in time in water. The other materials in this kit can be reused. This demonstration can reasonably be completed in one 50-minute class period.
• The holographic diffraction grating is very easily contaminated from oily or dirty hands. Wear latex, nitrile or polyethylene gloves when handling the diffraction grating. Fasten tape only to the outside edge of the diffraction grating. Store the diffraction grating in a plastic bag.
• If the overhead projector “leaks” light into the room, use a dark cloth to surround the base or the area where the light is “leaking.” Make sure not to cover the exhaust port of the overhead projector’s cooling fan.
• The darker the room, the better the resulting spectra. Remember that light intensity decreases with distance, so students in the back of the class may not be able to see as well as students in front of the class. Check the visibility of your setup before performing this activity. Some of the demonstrations may require students to stand close to the screen or wall in order to observe the spectra better. Perform this demonstration in an area that can accommodate your students if they need to move closer.
• The black sheets of construction paper can be cut to the appropriate size once the position of the slit is decided. Do not cut and size the paper until the optimal position of the slit on the overhead projector is determined (and therefore the position of the “selected” spectrum). The black sheets of paper need to cover the entire stage except for the 2-cm wide spacing for the slit. Depending on the size of the overhead projector stage, both sheets of paper can be used to create the slit, or one sheet can be cut with scissors or a paper cutter to produce the appropriately-sized sheets that will cover the stage and form the slit.
• Depending on the design of your overhead projector lens, the diffraction grating can be taped over the lens without modification. However, it can also be cut to fit the appropriate size and shape of the lens.
• Activity B: Other colors of food dye can be used for this demonstration.
• Activity C: The intensity of each primary color is not the same from an incandescent bulb in the overhead projector. For best results, position the “blue” mirror closest to the projector, the “green” mirror next and the “red” mirror the farthest from the projector. This arrangement will also give the students room to reflect the light without being in each other’s way.
• Activity C: If the intensity of the reflected light is too low when reflecting to a nearby wall, a support stand and clamp can be used to hold the white paper reflecting target. The target can then be positioned close to the reflecting mirrors to improve the brightness of the blended colors on the white paper.
• Holographic diffraction gratings are different from normal diffraction gratings in that instead of light traveling through tiny slits and diffracting, the light diffracts when it encounters varying amounts of phase delay when it travels through the specially designed grating. A holographic diffraction grating is highly efficient because virtually none of the light is blocked or scattered. The main importance for using the holographic diffraction gratings for these activities is they are highly dispersive and create a wider spectrum compared to a prism or normal diffraction grating. And unlike a prism, a diffraction grating (holographic or normal) disperses light in a linear fashion so the colors in the spectrum are spaced properly according to the wavelength. A spectrum projected from a prism will be non-linear with a large blue-green area while the red, orange and yellow will be squashed into one end. For more information on holographic diffraction gratings refer to the article in The Physics Teacher cited at the end of this activity.

Further Extensions

• Students can inspect the color of a television set at home with a magnifier to examine the light in each pixel. They will notice that three colors will be visible in an area that is white on the television, but the lights will be at different intensities. The students can examine how other colors like brown and pink are created by the varied intensity of the blue, red and green light.
• Students can take home a small piece of diffraction grating and observe various sources of light. They should observe the colors emitted from street lights, LEDs, the night sky including the moon, stars and other planets, the TV, incandescent lights versus fluorescent lights, etc. Make sure students do not use the diffraction grating to look directly at the Sun. Have students report their observations the next day.

Discussion

White light is composed of wavelengths of light from the visible spectrum as well as light wavelengths that are invisible to our eyes (i.e., infrared and ultraviolet). Classically, the visible spectrum has been called “ROY G BIV” after the various colors of light that are produced when white light is transmitted through a prism: Red, Orange, Yellow, Green, Blue, Indigo and Violet. Actually, there are an infinite number of colors in the visible spectrum range (see Figure 4).

The color of a piece of paper (or any object) results from the reflection of light from that paper. “Red” paper appears red when exposed to white light because the “red-colored” light waves that compose white light are reflected from the surface of the paper and the other wavelengths (e.g., blue, green, violet, orange) are absorbed. The red wavelengths of light reflect back to our eyes and interact with the cones in the retinas of our eyes. The cones are the color receptors in our eye. Our brain receives the signals sent from these cones and interprets that our eyes are seeing the color red. However, most materials do not reflect a pure single wavelength color and absorb all the other wavelengths. Instead, most materials reflect and absorb a combination of colors from the visible spectrum, which is what gives objects their distinct color. For example, a yellow shirt can appear yellow due to the reflection of mostly yellow wavelengths of light or it can appear yellow because a mixture of wavelengths (red and green) reflect from a surface and are interpreted by your brain to be the color yellow. Blending wavelengths and intensities of light generate an enormous variety of colors.

An important point about the color of an object is that an object can only reflect light wavelengths that are present in the light that illuminates it. That is, an object that is blue under white light will not appear blue under pure red light because there is no blue light to reflect. The color of the object depends on the light source. As mentioned above, most objects do not reflect only one wavelength of light and absorb all others—they reflect and absorb a combination of colors. Therefore, an object that appears blue under white light may still reflect other colors such as red and green light. However, the brightness of the blue reflection is much higher than the intensity of the other reflected colors so they have a very small affect on the observed color. These low intensity reflected colors can be observed when the dominantly reflected color of light is removed from the illuminating light source. When red light illuminates an object that appears blue under white light, the object appears reddish. It is important to mention that the object has always reflected red light even when it is illuminated by white light. However, since no blue light illuminates the object, there is no longer an overwhelming blue reflection. The minor red reflection is now seen and the object looks red. (Note: If the “blue” object absorbed, rather than reflected, all of the red light, the object would actually appear black when illuminated by the red light). This phenomenon is observed when the colored paper is placed into the projected spectrum. Colors of light that illuminate the paper that are not the “natural” color appear very dull, dark, or nearly black, whereas the “natural” color is very bright (the “natural” color being the color the object appears under normal white light). For example, the red paper looks dark blue and pale in the blue sector of the spectrum, and dull green in the green area of the spectrum. In the red light, the paper is very bright red. This demonstrates that the color pigments in the “red” paper reflect more than just red wavelengths. The pigments reflect the red wavelengths more intensely than the other colors, but the other colors are still reflected. An everyday example of this occurrence is seen with a candle flame. A candle flame produces light that has brighter yellow wavelengths of light than reds and blues so objects illuminated by a candle flame appear more yellowish.

A common misconception associated with color is the primary colors. Primary colors are colors that can not be formed by mixing other colors together. The primary colors of light are red, green and blue. These are often confused with the primary colors of paint (pigments) which are red, yellow and blue. The reason for the discrepancy is because mixing paint follows a different pathway than mixing light. The color observed from the process of mixing paint is the result of color mixing by subtraction. Blending colors of light form a color resulting from a process known as color mixing by addition.

Color mixing by subtraction occurs when the light illuminating an object is modified by the object before it reflects back to our eyes. That is, some wavelengths of the illuminating light are absorbed. Paint pigments absorb and reflect a combination of light wavelengths. Blue-paint pigments reflect not only blue light but also violet and green (at lower intensities), and absorb the other colors. The pigments in yellow paint reflect mostly yellow light, and to a lower degree red, orange and green wavelengths of light. Therefore, when blue paint and yellow paint are mixed and illuminated by white light, the blue pigments absorb red, orange, and yellow wavelengths (and the wavelengths in-between), and the yellow pigments absorb blue and violet wavelengths (and the wavelengths in between). Therefore, the only wavelengths that are not absorbed by either pigment are the green-colored wavelengths. The two pigments from the blue and yellow paint have subtracted (absorbed) all the colors from white light except for green. This explains why mixing blue and yellow paint results in the color green. The so-called primary colors of paint are actually not true primary colors. The best colors to use when mixing by subtraction are magenta (bluish-red), yellow and cyan (greenish-blue). These are known as the subtractive primary colors.

Color mixing by addition takes place when the blending light is unmodified when it illuminates an object and reflects back to our eyes. This is observed when two colors of light superimpose on a white sheet of paper. The white sheet reflects both wavelengths (actually all wavelengths) of visible light so they reflect back to our eyes unaltered. The different wavelengths of light add together and our brain recognizes this “combination” wavelength as another color. For example, equal amounts of red-light wavelengths and green-light wavelengths add together and appear yellow. When equal amounts of red, green and blue wavelengths of light are mixed, no color is seen. That is, white light is observed. An interesting property of yellow light and blue light is that when equal amounts of these wavelengths of light are added together, again white light is displayed. Yellow wavelengths and blue wavelengths of light are known as complementary colors, found directly across from each other on Newton’s Color Wheel (see Figure 5). Complementary colors are two colors that, when blended together, appear as white light. This occurs because the perception of the color “yellow” by our brain is the result of either ordinary yellow wavelengths of light reflecting to our eyes, or the result of the blending of red and green wavelengths of light. Our brain cannot tell the difference. Therefore, when equal amounts of yellow and blue light blend together, our brain interprets this combination of wavelengths the same as it would if equal amounts of red-, green- and blue-light wavelengths were mixed—resulting in white light. Magenta and green, and cyan and red are also complementary colors of light (see Figure 5).

References

Flinn Scientific would like to thank Doug De La Matter from Madawaska Valley D.H.S., Barry’s Bay, Ontario, for providing us with this “colorful” and “enlightening” activity.

Hewitt, Paul G. Conceptual Physics, 3rd Ed.; Addison Wesley Longman: Menlo Park, California, 1999; pp 422–431.

Sadler, Philip. “Projecting Spectra for Classroom Investigations”; The Physics Teacher; October 1991; pp 423–427.