In Investigation Four, students explored the spectrum of visible light using prisms and color filters.

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Ask students to review the terms used in the Investigation. Instruct students to refer to their Scientist’s Glossary if necessary.

1. Review the definitions of the terms that describe the physical characteristics of a light wave.

a. Ask students: What is the definition of a wave? A wave is the physical form of light energy.

b. Ask students: What is the definition of the highest point of a light wave? The highest point of a light wave is the crest.

c. Ask students: What is the definition of the lowest point of a light wave? The lowest point of a light wave is the trough.

d. Ask students: What is the definition of the distance between two crests or two troughs? The wavelength.

e. Ask students: How would you define the frequency of a light wave? The frequency is the number of crests or troughs of a light wave that occur in a certain time period.

f. Ask students: What is the relationship between the wavelength and the frequency of a light wave? There is an inverse relationship between the wavelength and the frequency of a light wave. As the wavelength increases, the frequency decreases.

2. Review the definitions of the terms that describe how a light wave interacts with matter:

a. Ask students: What occurs when light is absorbed by an object? Student answers will vary. The light wave is taken in by the object.

b. Ask students: What occurs when light is transmitted through an object? Student answers will vary. The light wave passes through the object.

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B. Discuss students’ observations from Trial 1. Ask students to refer to Problem 1 in their Student Data Record.

1. Begin a discussion of the light produced by the flashlight.

a. Ask students: Describe the light produced by the flashlight. The light is white light.

b. Ask students: Does the light contain the colors of the Visible Spectrum? Student answers will vary. Yes, the colors of the Visible Spectrum are contained in the light from the flashlight.

c. Ask students: How do your results demonstrate that the light from the flashlight contains the colors of the Visible Spectrum? Student answers will vary.

2. Discuss the color of light that was transmitted through the single-colored filters. Students should refer to Problems 1b through 1d.

Ask students: What was the color of light that was transmitted through the red filter? Students may suggest that only a single “red” wavelength of light was transmitted through the filter. 

a. Ask students: What color light was absorbed by the red filter? Student answers will vary. Students will likely suggest that wavelengths of light corresponding to colors of the Visible Spectrum other than red were absorbed.  Accept these types of answers during this initial analysis of Trial 1. A discussion of whether ONLY “red” light was transmitted and all other wavelengths of light were absorbed will occur later in the analysis.

b. Ask students: What was the color of light that was transmitted through the blue filter? Students may suggest that only a single “blue” wavelength of light was transmitted through the filter.

Ask students: What color light was absorbed by the blue filter? Students will likely suggest that wavelengths of light corresponding to colors of the Visible Spectrum other than blue were absorbed. Accept these types of answers during this initial analysis of Trial 1. A discussion of whether ONLY “blue” light was transmitted and all other wavelengths of light were absorbed will occur later in the analysis.

c. What was the color of light that was transmitted through the green filter? Green. 

Ask students: What color light was absorbed by the green filter? Students may suggest that only a single “green” wavelength of light was transmitted through the filter. Students will likely suggest that wavelengths of light corresponding to colors of the Visible Spectrum other than green were absorbed. Accept these types of answers during this initial analysis of Trial 1. A discussion of whether ONLY “green” light was transmitted and all other wavelengths of light were absorbed will occur later in the analysis.

d. Inform students that the absorption of the colors by each of the filters is due to the colored pigments in each of the filters. The red pigments absorbed all wavelengths except for those that combined to create the red color they saw; the blue pigments absorbed all wavelengths except for those that combined to create the blue color they saw, and the green pigments absorbed all wavelengths except for those that combined to create the green color they saw.

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3. Discuss whether only a single wavelength of light passed through each of the filters by analyzing students results from the remainder of Trial 1 when students combined the colored filters. Students should refer to Problems 1e through 1h of their Student Data Record.

a. Ask students: When you held the red and blue filters together in the light from the flashlight, what color light was transmitted onto the paper? Student answers will vary depending upon their perception of color. Students will likely describe the color of light as a purplish-red color, or magenta.

b. Ask students: When you held the red and green filters together in the light from the flashlight, what color light was transmitted onto the paper? Student answers will vary depending upon their perception of color. Students will likely describe the light as reddish in color.

c. Ask students: When you held the blue and green filters together in the light from the flashlight, what color light was transmitted onto the paper? Student answers will vary depending upon their perception of color. Students will likely describe the light as having a bluish/green or cyan color.

d. Ask students: Review your results from Trial 1 only. Do you think that each filter only allowed a single wavelength of light to pass through? What support do you have for your conclusion? Students should suggest that more than a single wavelength of light was transmitted through each filter because when the filters were combined, light of a certain color appeared. If only a single wavelength of light was transmitted through each filter, then when the filters were combined, no visible light would have been seen on the paper because the single wavelength transmitted through the first filter would have been absorbed by the second filter.

e. Ask students: Do you have data from another trial that would help you answer this question? Which trial? Students should suggest that data from Trials 3 and 4 could help to answer this question.

f. Tell students that they will return to this question again, after discussing their data from Trials 2, 3, and 4.

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C. Discuss students’ observations from Trial 2. Ask students to refer to Problem 2 in their Student Data Record.

1. Ask students: Describe the light produced by the flashlight. The light is white light.

2. Ask students: What happened to the light from the flashlight when it was shone through the prism? Student answers will vary. The light was separated into the colors of the Visible Spectrum.

3. Ask students: How did the prism separate the light from the flashlight? Student answers will vary. The prism separated the light by refracting the wavelengths of light of the Visible Spectrum into separate wavelengths that represent the colors of the Visible Spectrum.

4. Ask students: Describe the colors of the Visible Spectrum. Student answers will vary. The colors can be described as Violet / Indigo / Blue / Green / Yellow / Orange / Red. These colors are the most obvious of the colors of the Visible Spectrum and are most easily identified.

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D. Discuss students’ observations from Trial 3. Ask students to refer to Problem 3 in their Student Data Record. Students’ answers to the questions below may vary depending upon their perception of color. However, it is likely that their answers will reflect similarities to those described below. The answers to the questions are based upon sample data from Trial 3.

1. Ask students: Using just the prism and no filters, what color light fell on the sheet of paper? The light that fell on the paper was the Visible Spectrum.

2. Ask students: When the red filter was placed after the prism, what color light was transmitted through the prism onto the paper? Students should suggest that several different colors of light were visible. It is likely that they will describe the light that fell on the paper as red, dark blue, and violet colors, although the intense blue and violet may not have been as luminous as the red. 

Ask students: When the red filter was placed after the prism, what colors were missing from the Visible Spectrum? Many colors were missing such as those corresponding to green, yellow, and orange wavelengths. 

Ask students: Why? The “red” filter absorbed some of the wavelengths of the Visible Spectrum but permitted the transmission of wavelengths that corresponded to deep red, blue, and violet colors.

3. Ask students: When the blue filter was placed after the prism, what color light was transmitted through the prism onto the paper? Blue, green, cyan, and reddish-orange colors. 

Ask students: What colors were missing from the Visible Spectrum? Many colors were missing such as violet, and yellow. 

Ask students: Why? The “blue” filter absorbed almost all wavelengths of light except those corresponding to blue, green, reddish-orange, and cyan colors. Light of these wavelengths was transmitted through the filter.

4. Ask students: When the green filter was placed after the prism, what color light was transmitted through the prism onto the paper? Green, yellow, and orange. Although students may have had difficulty observing both the orange and yellow colors, they should have observed at least one other color in addition to the green color. 

Ask students: What colors were missing from the Visible Spectrum? Many colors were missing. 

Ask students: Why? The “green” filter absorbed all wavelengths of light except those that corresponded to green, yellow, and orange colors. These wavelengths of light were transmitted through the filter.

5. Remind students that the pigment in each of the filters absorbs all wavelengths of light except for those wavelengths that are transmitted.

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E. Discuss students’ observations from Trial 4. Ask students to refer to Problem 4 in their Student Data Record. Students’ observations may vary depending upon their perception of color. However, it is likely that their answers will reflect similarities to those described below. The answers to the questions are based upon sample data from Trial 4.

1. Ask students: When you held the red and blue filters together after the prism, what color light was transmitted through the two filters onto the paper? Students will likely describe the light as composed of violet and red colors.

2. Ask students: When you held the red and green filters together after the prism, what color light was transmitted through the two filters onto the paper? Students will likely describe the color of light as red.

3. Ask students: When you held the green and blue filters together after the prism, what color light was transmitted through the two filters onto the paper? Students will likely describe the light as composed of greenish-blue (cyan) and orange colors.

4. Ask students: How did combining two filters affect the colors of light that appeared on the paper? The first colored filter transmitted only certain wavelengths of light. Some, but not all, of these were absorbed by the second filter. Some were transmitted through the second filter. The colors that appeared on the paper were those that were transmitted through BOTH the first and second filters. All other wavelengths of light were absorbed by either the first or second filter. For example, the “red filter” transmitted red, blue, and violet colors of light and absorbed all other colors of light. The “blue filter” absorbed all colors of light except blue, green, cyan, and reddish-orange wavelengths. As a result, when the two filters were combined, only light of red, violet, and blue wavelengths was transmitted.

5. Ask students: Think back to Trial 1 when light from the flashlight passed through two filters. Can you explain why you saw the colors you observed when you combined the red and blue filters? The red and green filters? The blue and green filters? Use the data from Trials 3 and 4 to support your explanation. Students should record their answers in Problem 4f in their Student Data Record. Using the prism and the filters provided information about the different wavelengths of light transmitted by each filter. Rather than only one wavelength, each filter transmitted several different wavelengths of light. The first colored filter transmitted only certain wavelengths of light. Some, but not all, of these were absorbed by the second filter. Some were transmitted through the second filter. The colors that appeared on the paper were those that were transmitted through BOTH the first and second filters. All other wavelengths of light were absorbed by either the first or second filter. The color perceived on the paper was the combination of the wavelengths of light that passed through both filters.

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Note to Teacher: The following are extended explanations for the multiple filter experiments:

Red and blue filters

The “red filter” transmitted red, blue, and violet colors of light and absorbed all other colors of light. The “blue filter” absorbed all colors of light except blue, green, cyan, and reddish-orange wavelengths. As a result, when the two filters were combined, only light of red, violet, and blue wavelengths were transmitted. The combination of these wavelengths produced a light of magenta color.

Red and green filters 

The “red filter” transmitted red. blue and violet colors of light and absorbed all other colors of light. The “green filter” absorbed all colors of light except yellow and orange wavelengths. As a result, when the two filters were combined, only light of red and reddish-orange wavelengths was transmitted. The combination of these wavelengths produced a light of reddish color.

Green and blue filters

The “blue filter” absorbed all colors of light except blue, green, cyan, and reddish-orange wavelengths. The “green filter” absorbed all colors of light except yellow and orange wavelengths. As a result, when the two filters were combined, the combination of the wavelengths of light transmitted through both filters was of cyan color.

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F. Based on students’ observations throughout the Investigation, discuss the relationship between transmission and absorption of wavelengths of light by the transparent filters.

1. Wavelengths of the Visible Spectrum corresponding to the color of the filter are transmitted resulting in colored light that is the same as the color of the filter.

2. All other wavelengths of the Visible Spectrum are absorbed by the filter.

3. The pigments in each of the red, blue, and green filters absorb all of the light waves except for the light waves that are the same color as the pigments.

Cognitive Tool: Rules 

Ask students, Based on your observations, what rule describes the wavelengths of light transmitted through the filter and absorbed by the filter? Some light waves are absorbed and some are transmitted. Those light waves that are not absorbed by a colored filter are transmitted by the filter. The color of the filter corresponds to the wavelengths that are transmitted. Every other wavelength of light in the Visible Spectrum will be absorbed by the filter so that colors corresponding to these wavelengths are not represented in the light that passes through the filter.

For example, the red filter absorbed all other wavelengths of light (colors) in the Visible Spectrum apart from wavelengths corresponding to red, dark blue, and violet colors. The “red”, “dark blue” and “violet” wavelengths passed through the filter. When the light of these wavelengths combined, we perceived the color like a shade of red. This is why the filter appeared as a shade of red and the light seen on the paper appeared as a red shade.

Encourage students to use examples such as the one above as they record their answer to Problem 5 on the Student Data Record.

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Imagine how exciting it must have been to realize, through experiments very similar to those conducted by students in this Investigation, that the white light we see from the Sun and other sources is actually comprised of many different colors of light! And that you can separate and study the different wavelengths of light individually. Characterizing this phenomenon was just one of Isaac Newton’s many contributions to science.