Blackbody Radiation OBJECTIVES

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1 Name Class Date Skills Practice Lab Blackbody Radiation A perfect absorber of radiation also happens to be a perfect radiator of that radiation as well. Such objects are called blackbodies, because darker objects tend to absorb (and therefore radiate) energy better than light objects, and the best absorber/radiator is a perfectly black object. In the late 19th century, blackbodies were studied to gain better understanding of the properties of electromagnetic radiation, which includes radio waves, infrared radiation, ultraviolet radiation, and x-rays, as well as visible light. As a blackbody is heated, it emits radiation that is both brighter and of shorter wavelength (that is, more toward the blue end of the spectrum). Although not blackbodies in the strictest sense, stars and other incandescent objects simulate blackbody behavior. Therefore, the hotter these objects are, the brighter and bluer is the light that they emit. In this lab, you will observe the blackbody behavior of two incandescent light bulbs, and how the light from them changes as the electric current increases and the filament in the bulb becomes hotter. You will then apply this information to make predictions about the temperature and brightness of stars. OBJECTIVES Using Scientific Methods Model blackbody behavior using an incandescent bulb. Using Scientific Methods Observe changes in the light as electric current is varied. Apply the results of the lab to the colors and brightness of stars. Draw conclusions about the nature of those stars. MATERIALS electric lamp with threaded socket (can accept light bulbs up to 100 W) incandescent light bulb, 50 W, 1 incandescent light bulb, 100 W, 1 variable resistor, alternating current, 110 V SAFETY PROCEDURE 1. Insert the 50 W light bulb into the socket of the lamp, and plug the lamp into the outlet of the variable resistor. CAUTION: Handle light bulbs with care. They can break easily and cut skin. Touching a hot light bulb may cause serious burns. Holt Earth Science 49 Stars, Galaxies and the Universe

2 Name Class Date 2. Check to be sure the variable resistor unit is turned off, and that the dial is set to zero. Plug the resistor into a ground fault circuit interruptor (GFCI) protected circuit. 3. Turn the variable resistor on, then switch the lamp on. Gradually turn the dial of the resistor until the lamp begins to glow. 4. Note the color and brightness of the filament of the lamp. Carefully place your hand near the bulb without touching the glass surface. Note whether the glass is warm or cool. Write down your observations in Table 1, below. 5. Increase the setting of the variable resistor to the low position. Again note the color and brightness of the lamp s filament. Carefully place your hand near the bulb without touching it, and note whether it is cool, warm, or hot. Write down your observations in Table Repeat Step 5, increasing the setting of the resistor first to medium, and then to medium-high. The bulb should appear very bright. CAUTION: Do not turn the setting all the way up to high. TABLE 1 50 W LIGHT BULB Variable Resistor Setting Color of Filament Relative Brightness of Filament Relative Temperature of Bulb 7. Turn the variable resistor dial back to zero, and switch off both the lamp and the resistor. Wait until the bulb has cooled sufficiently, then remove it from the lamp and replace it with the 100 W light bulb. Holt Earth Science 50 Stars, Galaxies and the Universe

3 Name Class Date 8. Repeat Steps 1 through 6 using the 100 W light bulb. Record your observations in Table 2. TABLE W LIGHT BULB Variable Resistor Setting Color of Filament Relative Brightness of Filament Relative Temperature of Bulb 9. Turn the variable resistor dial back to zero, and switch off both the lamp and the resistor. Unplug the resistor from the outlet and the lamp from the resistor. CAUTION: The bulb will probably be very hot; therefore, do not remove the bulb from the lamp. ANALYSIS AND CONCLUSION 1. Evaluating Methods Using only your eyes, you have been able to differentiate between the colors and brightness of the light bulbs at the different variable resistor settings. What other methods could be used to make these observations? 2. Evaluating Results What differences did you observe in the light from the two different light bulbs? What conclusions can you draw from your observations? Holt Earth Science 51 Stars, Galaxies and the Universe

4 Name Class Date 3. Making Predictions Certain stars in the night sky appear red or orange, while others appear yellow, white, or blue-white. From your observations, what can you state about these stars? 4. Making Predictions Certain red stars are as bright or brighter than certain blue stars. Explain why this does not contradict your observations. EXTENSION 1. Applying Conclusions If the equipment is available, obtain a prism or grating spectroscope and repeat the lab. For each setting of the variable resistor, observe the spectrum of the light, and notice if it is brighter in the red, yellow, or blue part of the spectrum. Record your observations below, and note whether the added information helps you to draw more exact conclusions. Holt Earth Science 52 Stars, Galaxies and the Universe

5 Skills Practice Lab Blackbody Radiation Teacher s Notes TIME REQUIRED one 45-minute class period LAB RATINGS Teacher Prep 2 Student Set-Up 2 Concept Level 3 Clean Up 1 Easy Hard Gabrielle Schavran Western Rockingham Syosset High School Syosset, New York SKILLS ACQUIRED Constructing Models Experimenting Inferring Predicting THE SCIENTIFIC METHOD In this lab, students will Make Observations Draw Conclusions MATERIALS Depending on how many variable resistors are available, each lab station may have between two and four students. CAUTION: Use ground fault circuit interruptor (GFCI) protected circuits when performing this lab, and make sure students do not work on the lab near water. Students should exercise caution when operating electrical equipment, as well as when handling the bulbs to prevent breakage. Be sure students do not touch the bulbs with bare hands while performing this lab. Holt Earth Science 75 Stars, Galaxies and the Universe

6 TIPS AND TRICKS Students should gradually increase the resistance until the dial is only about halfway to its maximum setting. If too much current is passed through the bulb, the bulb will burn out. Be sure to have extra bulbs on hand in case of burnouts or breakage. If you have several hand-held diffraction gratings available, you may wish to distribute these to students and have them try to determine whether the spectrum of the light for a given setting is brighter in the red or blue regions. Students may also be able to determine whether brighter settings are much brighter than dimmer settings in the blue portion of the spectrum. Holt Earth Science 76 Stars, Galaxies and the Universe

7 Name Class Date Skills Practice Lab Blackbody Radiation A perfect absorber of radiation also happens to be a perfect radiator of that radiation as well. Such objects are called blackbodies, because darker objects tend to absorb (and therefore radiate) energy better than light objects, and the best absorber/radiator is a perfectly black object. In the late 19th century, blackbodies were studied to gain better understanding of the properties of electromagnetic radiation, which includes radio waves, infrared radiation, ultraviolet radiation, and x-rays, as well as visible light. As a blackbody is heated, it emits radiation that is both brighter and of shorter wavelength (that is, more toward the blue end of the spectrum). Although not blackbodies in the strictest sense, stars and other incandescent objects simulate blackbody behavior. Therefore, the hotter these objects are, the brighter and bluer is the light that they emit. In this lab, you will observe the blackbody behavior of two incandescent light bulbs, and how the light from them changes as the electric current increases and the filament in the bulb becomes hotter. You will then apply this information to make predictions about the temperature and brightness of stars. OBJECTIVES Using Scientific Methods Model blackbody behavior using an incandescent bulb. Using Scientific Methods Observe changes in the light as electric current is varied. Apply the results of the lab to the colors and brightness of stars. Draw conclusions about the nature of those stars. MATERIALS electric lamp with threaded socket (can accept light bulbs up to 100 W) incandescent light bulb, 50 W, 1 incandescent light bulb, 100 W, 1 variable resistor, alternating current, 110 V SAFETY PROCEDURE 1. Insert the 50 W light bulb into the socket of the lamp, and plug the lamp into the outlet of the variable resistor. CAUTION: Handle light bulbs with care. They can break easily and cut skin. Touching a hot light bulb may cause serious burns. Holt Earth Science 49 Stars, Galaxies and the Universe 77

8 Name Class Date 2. Check to be sure the variable resistor unit is turned off, and that the dial is set to zero. Plug the resistor into a ground fault circuit interruptor (GFCI) protected circuit. 3. Turn the variable resistor on, then switch the lamp on. Gradually turn the dial of the resistor until the lamp begins to glow. 4. Note the color and brightness of the filament of the lamp. Carefully place your hand near the bulb without touching the glass surface. Note whether the glass is warm or cool. Write down your observations in Table 1, below. 5. Increase the setting of the variable resistor to the low position. Again note the color and brightness of the lamp s filament. Carefully place your hand near the bulb without touching it, and note whether it is cool, warm, or hot. Write down your observations in Table Repeat Step 5, increasing the setting of the resistor first to medium, and then to medium-high. The bulb should appear very bright. CAUTION: Do not turn the setting all the way up to high. TABLE 1 50 W LIGHT BULB Variable Resistor Setting Color of Filament Relative Brightness of Filament Relative Temperature of Bulb 7. Turn the variable resistor dial back to zero, and switch off both the lamp and the resistor. Wait until the bulb has cooled sufficiently, then remove it from the lamp and replace it with the 100 W light bulb. Holt Earth Science 50 Stars, Galaxies and the Universe 78

9 Name Class Date 8. Repeat Steps 1 through 6 using the 100 W light bulb. Record your observations in Table 2. TABLE W LIGHT BULB Variable Resistor Setting Color of Filament Relative Brightness of Filament Relative Temperature of Bulb 9. Turn the variable resistor dial back to zero, and switch off both the lamp and the resistor. Unplug the resistor from the outlet and the lamp from the resistor. CAUTION: The bulb will probably be very hot; therefore, do not remove the bulb from the lamp. ANALYSIS AND CONCLUSION 1. Evaluating Methods Using only your eyes, you have been able to differentiate between the colors and brightness of the light bulbs at the different variable resistor settings. What other methods could be used to make these observations? Answers may vary. Sample answer: These observations could be also be made by using instruments, such as spectroscopes or photometers, to measure differences in the bulbs color and brightness. 2. Evaluating Results What differences did you observe in the light from the two different light bulbs? What conclusions can you draw from your observations? Answers may vary. Sample answer: The more current that passes through the filament, the hotter it becomes, and the brighter and bluer the light it emits becomes. Holt Earth Science Stars, Galaxies and the Universe

10 Name Class Date 3. Making Predictions Certain stars in the night sky appear red or orange, while others appear yellow, white, or blue-white. From your observations, what can you state about these stars? Answers may vary. Sample answer: The redder stars are cooler than the whiter and bluer stars. However, the brightness of the stars depends on factors other than the star s temperature. 4. Making Predictions Certain red stars are as bright or brighter than certain blue stars. Explain why this does not contradict your observations. Answers may vary. Sample answer: The redder stars may be closer than the blue stars, and therefore they appear brighter. The red stars may also be red giants or red supergiants, which are brighter because they are much larger than the other stars. EXTENSION 1. Applying Conclusions If the equipment is available, obtain a prism or grating spectroscope and repeat the lab. For each setting of the variable resistor, observe the spectrum of the light, and notice if it is brighter in the red, yellow, or blue part of the spectrum. Record your observations below, and note whether the added information helps you to draw more exact conclusions. Answer may vary. Holt Earth Science 52 Stars, Galaxies and the Universe 80

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