Laser Simulation-Visual Light Lab for High School Students

Transcription

1 Laser Simulation-Visual Light Lab for High School Students Marcus McAleer Pullman High School Pullman, WA & Lucinda Mendenhall Lewis-Clark State College Lewiston, ID Washington State University Mentors Dr. Gary Cheng Mechanical and Materials Engineering & Muhammad Daniel Pirzada Graduate Student July, 2005 The project herein was supported by the National Science Foundation Grant No. EEC : Dr. Richard I. Zollars, Principal Investigator and Dr. Donald C. Orlich, co PI. The module was developed by the authors and does not necessarily represent an official endorsement by the National Science Foundation.

2 Summary & Rationale: The purpose of this teaching module is to give students a hands-on introductory experience involving the science and engineering applications of lasers and light of various frequencies. This will include the topics of light, color, energy, heat, and absorption. The students will investigate how different objects absorb energy at different frequencies and then use their observations to make predictions and draw conclusions. The intended audience of this module is early high school students but this can easily be modified for students ranging from 6 th to 12 th grade. The module can be modified to meet the needs of either a science class or a math class (Algebra or Statistics). Each of the four lessons can be accomplished in one 60 minute class period, but can easily be extended for longer blocked periods. Students do not need much prior knowledge for this module. Students should know that white light is a combination of all colors of light, however a quick demonstration of refraction of sunlight with a prism will establish this fact. Students also need to know how to draw a line graph. Introduction To Lasers & Applications: LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Einstein was the first to predict that you could have a stimulated emission of radiation in This was based on the then new theory that the energy levels in atoms were not continuous but discrete. Einstein predicted that when an electron dropped from one energy state to a lower one, a photon containing that specific discrete drop in energy could be emitted. If the energy level of the emitted photons was that of visible light (between 400 and 700 nanometers in wavelength), then we should be able to see this 1

3 emission of photons as light. This emission of photons could be captured and projected as a beam by using parabolic mirrors. Furthermore, since the energy is very specific, light of one specific wavelength and thus of one specific color could be generated (see below). Also, the electron energy levels are dependent upon the element. Thus, using different elements to construct lasers would produce lasers of different colors. The first laser ever made was in 1960 which was a solid ruby laser. Since then, many other substances have been used to create a variety of laser light. The laser that most people are familiar with is a helium-neon laser that produces red light. The basic construction of a laser is shown below (the pumping system is the method for initially getting the electrons to a higher energy state). 2

4 Lasers produce a focused monochromatic beam of light which allows for a myriad of modern applications. Substances will either reflect, absorb or transmit energy at different wavelengths based on its atomic structure. If a substance absorbs energy at the same wavelength that a laser produces, then that laser can be used to generate heat in a very focused area. Thus, lasers are used in many machining processes in which the desired result is to have holes of exacting micro-dimensions in the substance. The type (color & intensity) of laser that is chosen is dependent upon the material being machined. A laser beam can also be used to melt substances and weld materials on a very small scale. Furthermore, a laser beam can be used in the crystallization of materials used in micro-processing. Medical applications include the ablation of certain tissues which include the now common laser eye surgery. Furthermore, non-invasive laser surgery is now being pursued in the removal of internal cancerous tumors. In these applications, a material (nano-particles) is attached to the tumor (via anti-bodies) and then a laser beam is targeted on the tumor. The color of the laser is chosen in such a way that the skin of the person transmits the specific wavelength of the beam, but the nano-particles absorb it and change the energy into heat. This heat then destroys the tumor without much effect upon the surrounding tissue. Classroom Applications & Module Overview: Unfortunately, these applications are not easily shown in the classroom. The lasers used in these applications are expensive and powerful and would most likely introduce a safety hazard for students. Common lasers (like laser pointers) simply do not have enough power to generate significant amounts of heat to be a useful tool for these 3

5 applications. However, the principles involved can still be investigated by simply focusing high intensity color-filtered light. Various objects can then be placed at the focal point and data can be collected. The data will be in the form of simple visual observations and also temperature recordings of these materials subjected to the focused light. The first lesson will demonstrate how focused white light affects only the small region of the material where the light is focused while leaving the remaining parts unaffected. The subsequent lessons will demonstrate the effects that colored light has on various objects. To create an apparatus that generates focused colored light will require the following: High powered light source Platform for spotlight with hole in center Colored filters Large focusing lens Platform for lens with hole in it. Stacking materials (wood/brick blocks) Method for holding sample at focus point For our first apparatus (which we called the laser simulator), we used a 6,000,000 candlepower portable halogen spotlight for our light source. This was suspended over a small chair with the seat removed. The chair was elevated by bricks. At a certain distance below the chair, a platform that contained a lens was placed. A hole was cut out of this platform to hold the lens. We used small 2 ½ pound weights (from the PE 4

6 department) to hold our samples at the focal point. These were chosen because they allowed us to suspend our samples over the hole in the weights and also because we could adjust the height easily by adding or subtracting weights. The heights needed to be adjusted based on the thickness of our samples. The colored filters we used colored transparent clip boards that we purchased from various office supply stores. These could be either placed on the chair and be used also as a support to the spot light, or they could be placed directly over the lens. We also used small rounded color filters used for stage lighting. We constructed our second laser simulator out of wood so that we didn t need the chair or the bricks. The distances from the light source to the lens and from the lens to the focal point will depend on the lens that you use and should be determined before you build your simulator. Laser Simulator #1 Lens & Weights Laser Simulator #2 Candle Melting 5

7 Considerations, Limitations, & Extensions: Unfortunately, you cannot exactly obtain the atomic absorption spectrum of a substance in this way. The atomic absorption spectrum of a substance is based on the energy levels of its electrons. Using the laser simulator, you can instead determine the macro absorption spectrum of a substance which is also a function of its molecular structure and its specific heat and is revealed by its color. If an object is a certain color, that means that it is reflecting light of that particular wavelength and absorbing light of the opposite color. For example, the opposite of blue is yellow and thus a blue object is absorbing yellow light and reflecting blue light. This module can easily be modified to emphasize the relationships of color, reflection, transmittance and absorption. However, the baseline that absorption in general depends upon the color of light for a substance will be firmly established. Unless your filters are professional quality, they probably will be flawed in two main ways. First, your different filters may allow different intensities of light to pass through. However, this can be accounted for by using a light sensor and then dividing your temperature data by the light intensity for each filter. In this way you can directly compare the different effects different colors have on your objects. A second problem will be that some of your filters will only block out the opposite of the transmitting color. For example, your blue filter may be only filtering out yellow light and allowing all other wavelengths to pass through. However, it will still look blue with the absence of yellow, but will be allowing red, orange, green and purple through. 6

8 Your light source may also pose a small problem. Our halogen spotlight only lasted 30 minutes on a battery charge and was unable to run from a normal wall socket. Thus we had to buy an adaptor that would allow us to run the spotlight continuously. Furthermore, halogen lights provide light that is not evenly distributed among the colors. Specifically, yellow light is more intense than any other color. Thus, our blue colored materials heated up more than any other color. This is similar to a problem with your filters and can be overcome in the same way by using a light sensor with each filter and calibrating your results. Another way to construct your laser simulator is to use a prism and mirrors to isolate the specific band of the spectrum that you desire and then run that color through your lens. This will give you only one color going through your lens and will avoid all problems with filters. However, this may be too difficult to actually construct without expensive equipment. Either way you design your apparatus, you will need ten of them so that each group of 3 can be all working at the same time. Obviously this may be cost prohibitive. If you do not have it in your budget to buy the needed equipment, then you may want to try to write a grant to obtain the money. If all you can obtain is equipment for one station, then you can work groups through the laser simulator over a longer period of time as you teach a whole unit. Although many of these issues may limit the level of accuracy in measuring the actual absorption spectrum of materials, the fact that the color of light matters when dealing with absorption of energy will be firmly established. To get more accurate spectral absorption data, a spectrometer should be used. In a spectrometer, white light of uniform 7

9 intensity across the spectrum is separated out into its color bands with a prism. These bands are then focused with mirrors to pass one at a time through a solution. A light intensity reading is taken before the light hits the solution, and another one is taken after the light passes through the solution. Comparing these two intensities yields the percentage of light that was absorbed by the solution. However, the drawback of using a spectrometer is that all of the action takes place inside the device out of sight of the students. Students simply put in a solution, press some buttons and then read the numbers. This type of magic box may impede the intuitive sense that students should develop regarding light absorption. The laser simulator, on the other hand, is extremely interactive, and although it may provide modest results, it is a great aid in developing student understanding of light, color, absorption, and laser applications. After this module is completed, a follow up lab with a spectrometer would be ideal and would supply more accurate data for further analysis. Acknowledgements: We would like to thank Dr. J. Michael Winey (WSU) for enlightening conversations regarding the physics involved in this module. We would also like to thank Mr. Norman Mahler for constructing the 2 nd laser simulator for us. 8

10 Lab #1-Focused Light Introduction: Did you ever play with a magnifying glass outside in the sunlight as a little kid? Today your group will briefly see the effects of focused light. As you proceed, make conjectures about what you see and discuss each others ideas. Materials needed: Plastic insect Egg white Spatula Small Glass Container Stop Watch Laser Simulator Safety: Never look directly into the light source. Do not place your hands at the focal point for more than 1 second when the light source is turned on. Wear your safety glasses at all times when the light source is turned on. Do not touch the material directly after being removed from the focused light. Always be attentive to what you & your partners are doing. 9

11 Procedure: Follow these steps each time an item is placed in the laser simulator. Make sure light source is off before placing material under light source. Place the object in the dish. Place the dish under light source. Turn on the light source. Quickly adjust the object if needed to make sure it is at the focal point. Start timer (if needed) Turn off light source at desired time. Analyze results. I. Introduction to Focused Light In this portion of the lab, you will see the effects of focused light on a plastic insect. A) What do you think will happen when a plastic insect is placed under the focused light? Explain your answer. Place plastic insect under focused light. 10

12 B) Describe what happened. Did it respond the way you thought it would? Why do you think you got the results you did? II. Focused Light & Heat In this portion you will be comparing the ideas of heat and focused light. A) What would cook or melt fastest in a frying pan, an egg or a plastic spatula? Which would cook or melt fastest, if at all, under the focused light? Explain. Place a dish of egg white under the focused light for 3 minutes. Place plastic spatula under the focused for allotted amount of time. B) Describe what happened. Did you expect these results? Why or why not? 11

13 C) Would lining the dish of egg with aluminum foil change these results? If so, how? Line a dish with aluminum foil. Fill dish with egg white. Place egg dish under the focused light for 3 minutes. D) Did lining the dish change your results? Offer an explanation for what you observed? III. Conclusion Does focused light generate heat? Offer an explanation for what you observed. 12

14 Teacher Guide Lab #1 Focused Light Purpose & Results: The purpose of this lab is to demonstrate that focused light can be transformed into heat. This focused heat just affects the small area of focused light without affecting the surrounding area. Furthermore, students will see for themselves examples of transmission of light (glass container & egg white), absorption of light (spatula & egg white), and reflection of light (tin foil). It may be surprising to the students that the spatula starts smoking almost immediately while the egg barely cooks. This is opposite of what happens in a frying pan. The purpose of the tin foil is the show that light and heat is reflected back through the egg and so more of it will cook. The insect also smokes immediately and serves the purpose of starting the module on a fun and exciting note. Technical Considerations: For best results, the chosen insect and the spatula should both be made of black plastic. These objects should not be left in the laser simulator for more than a few seconds because they will immediately start smoking. When the egg white is used, the glass dish used to contain it should only be filled to a depth of about ½ inch. If you put too much egg white in, the tin foil will have a smaller effect and the students may not notice the difference. 13

15 Discussion Points: During the lab, make sure that the students make predictions and discuss their thoughts behind their predictions with each other. Then, when they see the results, the can have rich discussions modifying their thinking. After the lab is complete, students should share their results with the class and a class discussion should follow. During this discussion, ask what advantage would there be in affecting a small area with laser light while not touching the surrounding areas. Students should realize that that this would be beneficial in medical applications such as laser eye surgery. You can give other examples (machining, welding, crystallization etc.). Then ask how the different objects (glass, egg white, tin foil, black plastic) responded differently to light. This will lead to a description of reflection, absorption, and transmission and will allow you to introduce the next lab concerning absorption and color. 14

16 Lab #2a Colored Candles Introduction: Today your group will see how candles of different color respond to focused light of various colors. As you proceed, make conjectures about what you see and discuss each others ideas. Materials needed: Small colored candles Wax paper or card board Laser Simulator Stop Watch Safety: Never look directly into the light source Do not place your hands at the focal point for more than 1 second when the light source is turned on. After every 30 minutes, turn the light source off for 5 minutes in order to cool down you can use this time to discuss your results Wear your sunglasses at all times when the light source is turned on Make sure the shield is in place when the light source is turned on Always be attentive to what you & your partners are doing 15

17 Procedure: Follow these steps every time that you test a candle. Make sure light source is off Place the desired candle in holder so that a fresh side is up Turn on the light source Quickly adjust your candle if needed to make sure the candle is at the focal point Start timer Put up shield After the allotted time, turn off the light source Remove the shield Remove the candle and dump any melted wax onto a piece of cardboard Analyze results I. No Filter In this portion of the lab, you will compare what happens to the dark green candle and the pink candle when these are placed underneath the lens while not using a colored filter. A) What do you expect to happen to these candles? Will any wax melt? Will both candles respond in the same way? Explain. 16

18 Place the green candle under the lens for 2.5 minutes. Place the pink candle underneath the lens for 2.5 minutes. B) Describe what happened. Did both candles respond in the same way? How did your results compare to your predictions? Why do you think this happened? II. Yellow Filter In this portion of the lab, you will compare what happens to the blue candle and the yellow candle when these are placed underneath the lens while using the yellow filter. A) What do you expect to happen to these candles? Which candle (if any) do you think will be melted the most by the yellow light? Explain your guesses. 17

19 Place the yellow filter on top of the lens. Place the yellow candle under the lens for 2.5 minutes. Place the blue candle underneath the lens for 2.5 minutes. B) Describe what happened. Did the candles respond in the same way? How did your results compare to your predictions? Why do you think this happened? III. Blue Filter In this portion of the lab, you will compare what happens to the blue candle and the yellow candle when these are placed underneath the lens while using the blue filter. A) What do you expect to happen to these candles? Which candle (if any) do you think will be melted the most by the blue light? Explain your guesses. 18

20 Place the blue filter on top of the lens. Place the yellow candle under the lens for 2.5 minutes. Place the blue candle underneath the lens for 2.5 minutes. B) Describe what happened. Did the candles respond in the same way? How did your results compare to your predictions? How did your results compare with what happened when the yellow filter was used? Why do you think this happened? 19

21 IV. Magenta Filter In this portion of the lab, you will compare what happens to the magenta candle, the green candle, and the white candle when these are placed underneath the lens while using the magenta filter. A) What do you expect to happen to these candles? Which candle (if any) do you think will be melted the most by the magenta light? Which candle (if any) do you think will be melted the least by the magenta light? Explain your guesses. Place the magenta filter on top of the lens. Place the magenta candle under the lens for 2.5 minutes. Place the green candle underneath the lens for 2.5 minutes. Place the white candle underneath the lens for 2.5 minutes. B) Describe what happened. Did the candles respond in the same way? How did your results compare to your predictions? Why do you think this happened? 20

22 V. Green Filter In this portion of the lab, you will compare what happens to the magenta candle, the blue candle, and the green candle when these are placed underneath the lens while using the green filter. A) What do you expect to happen to these candles? Which candle (if any) do you think will be melted the most by the green light? Which candle (if any) do you think will be melted the least by the green light? Explain your guesses. Place the green filter on top of the lens. Place the magenta candle under the lens for 2.5 minutes. 21

23 Place the blue candle underneath the lens for 2.5 minutes. Place the green candle underneath the lens for 2.5 minutes. B) Describe what happened. Did the candles respond in the same way? How did your results compare to your predictions? Why do you think this happened? How do these results compare to what happened when the magenta filter was used with these candles? Explain why this is. V. Conclusions A) Do different colors of light effect objects in the same way? 22

24 B) Try to generalize which colors of light effect which colors of candles. Offer an explanation for your generalization. C) Do you think your results would be different if we used a different light source? Explain. D) Compare your results with another group. 23

25 Lab #2b Colored Candles Introduction: Today your group will see how candles of different color respond to focused light of various colors. As you proceed, make conjectures about what you see and discuss each others ideas. Materials needed: Small colored candles Wax paper or card board Laser Simulator Safety: Never look directly into the light source Do not place your hands at the focal point for more than 1 second when the light source is turned on. After every 30 minutes, turn the light source off for 5 minutes in order to cool down you can use this time to discuss your results Wear your sunglasses at all times when the light source is turned on Make sure the shield is in place when the light source is turned on Always be attentive to what you & your partners are doing 24

26 Procedure: Follow these steps every time that you test a candle. Make sure light source is off Place the desired candle in holder so that a fresh side is up Turn on the light source Quickly adjust your candle if needed to make sure the candle is at the focal point Start timer Put up shield After the allotted time, turn off the light source Remove the shield Remove the candle and dump any melted wax onto a piece of cardboard Analyze results I. Yellow Filter In this portion of the lab, you will compare what happens to the red candle, the light blue candle, and the purple candle when these are placed underneath the lens while using the yellow filter. A) What do you expect to happen to these candles? Which candle (if any) do you think will be melted the most by the yellow light? Which candle (if any) do you think will be melted the least by the yellow light? Explain your guesses. 25

27 Place the yellow filter on top of the lens. Place the red candle under the lens for 2.5 minutes. Place the light blue candle underneath the lens for 2.5 minutes. Place the purple candle underneath the lens for 2.5 minutes. B) Describe what happened. Did the candles respond in the same way? How did your results compare to your predictions? List the candles in order from most melted to least. II. Blue Filter In this portion of the lab, you will compare what happens to the red candle, the light blue candle, and the purple candle when these are placed underneath the lens while using the blue filter. 26

28 A) What do you expect to happen to these candles? Which candle (if any) do you think will be melted the most by the blue light? Which candle (if any) do you think will be melted the least by the blue light? Explain your guesses. Place the blue filter on top of the lens. Place the red candle under the lens for 2.5 minutes. Place the light blue candle underneath the lens for 2.5 minutes. Place the purple candle underneath the lens for 2.5 minutes. B) Describe what happened. Did the candles respond in the same way? How did your results compare to your predictions? List the candles in order from most melted to least. How does this order compare to when the yellow filter was used? 27

29 III. Green Filter In this portion of the lab, you will compare what happens to the red candle, the light blue candle, and the purple candle when these are placed underneath the lens while using the green filter. A) What do you expect to happen to these candles? Which candle (if any) do you think will be melted the most by the green light? Which candle (if any) do you think will be melted the least by the green light? Explain your guesses. Place the green filter on top of the lens. Place the red candle under the lens for 2.5 minutes. Place the light blue candle underneath the lens for 2.5 minutes. Place the purple candle underneath the lens for 2.5 minutes. B) Describe what happened. Did the candles respond in the same way? How did your results compare to your predictions? List the candles in order from most melted to least. 28

30 IV. Magenta Filter In this portion of the lab, you will compare what happens to the red candle, the light blue candle, and the purple candle when these are placed underneath the lens while using the magenta filter. A) What do you expect to happen to these candles? Which candle (if any) do you think will be melted the most by the magenta light? Which candle (if any) do you think will be melted the least by the magenta light? Explain your guesses. Place the magenta filter on top of the lens. 29

31 Place the red candle under the lens for 2.5 minutes. Place the light blue candle underneath the lens for 2.5 minutes. Place the purple candle underneath the lens for 2.5 minutes. B) Describe what happened. Did the candles respond in the same way? How did your results compare to your predictions? List the candles in order from most melted to least. How does this order compare to when the green filter was used? V. Conclusions A) Do different colors of light effect objects in the same way? 30

32 B) Try to generalize which colors of light effect which colors of candles. Offer an explanation for your generalization. C) Do you think your results would be different if we used a different light source? Explain. D) Compare your results with another group. 31

33 Teacher Guide Lab #2 Colored Candles Purpose & Results: The purpose of this lab is to firmly establish that the amount of heat generated by focused light is dependent upon the color of the object being tested and the color of light being applied. Furthermore, a relationship between which colors absorb more light under what situations should start coming into focus for the students as they proceed through this lab. The lab has been broken down into two parts in order to establish two different points. The design is for half of the groups to do lab #2a, while the other groups do lab #2b. These two labs should take about the same amount of time. After all groups are done, a whole class discussion should be held in which students from both versions of the lab will get to discuss their results and make conjectures about what they observed. Another design would be to have each group do both labs over two days and then discuss their results. The purpose of #2a is to show that candles of the same color as the filter will not melt very much, while candles of the color compliment of the light color will melt the most. At first, with no filter, the green candle will melt more than the pink candle because the green that I was using was a dark hue and thus absorbed more. Also, my light source was intense in the red portion of the spectrum & thus affected the dark green candle more. However, students will often focus on the darkness of the green candle and always 32

34 assume that the darker candle will always absorb more light. This will also be true when the blue candle absorbs more than the yellow candle when the yellow filter is used. However, when they use the blue filter, the yellow candle will absorb more. This should puzzle the students as they have to discard their darkness conjecture and revise their thinking. The next two filters will yield similar results: the magenta filter will melt more of the green candle while the green filter will melt more of the magenta candle. The white candle should reflect all visible light and thus not be effected very much only a little due to infra red waves. The purpose of lab #2b is to establish that blue & yellow are opposite or complimentary colors, and that green and magenta are complimentary colors. When the students put the red, light blue & purple candles under the green light, the purple will absorb the most, followed by red blue, and then blue. However, when the magenta filter is used, the candles absorb in the exact opposite order. This should pique the students interest and might be a little surprising since purple is darker than green. The same thing happens with the blue and the yellow filters the blue melts the red the most, followed by the light blue & then the purple, while the yellow filter reverses this order. Thus, the idea of color opposites are developed in the students minds. 33

35 Technical Considerations: This lab is very sensitive to the following: Candle Color Filter Color Filter Purity Spectrum of Light Source Student Accuracy Ideally, the candle colors available to you will match the color of filters that you have and these colors should come in compliment pairs (Blue-Yellow; Green-Magenta; Red- Cyan). However, this may not be the case. If your colors are not perfect, try combinations beforehand to get the desired results. In our test module, we did not have a magenta candle but were able to establish that a red filter melted a green candle more than a red candle and that a green filter melted the red candle more than the green candle. Also, the filters we used did not just allow one color to transmit. Our filters only absorbed the compliment of the color. Thus, our yellow filter did not allow blue light through, but did allow everything else through. However, with the absence of yellow s compliment, the light looks yellow but it actually contains every color except blue. This can be confusing, because one might think that the yellow filter should only affect the blue candle. However, since green light is allowed through the yellow filter, a magenta candle will be affected etc. 34

36 The biggest technical consideration is the light source. Ideally you can obtain a high intensity light source that gives uniform intensity throughout the entire spectrum. We realized this after we purchased two high intensity halogen spot lights. Halogen lights give off a light that is very intense in the yellow portion of the spectrum (red is also strong) and not very intense in the green-blue-purple portion of the spectrum. This can be shown by placing a prism next to your light source and visually inspecting the intensity and the band width of each spectral color. If one color is much more intense than others, your results may become muddled. Thus, through prior experimentation, you have to design combinations of candles and filters that illustrate these principles best. With our halogen light, we could never melt the yellow candle very well and we didn t even do the portion with the blue filter and the yellow candle. Once students understand what is going on, this non-uniform intensity can actually add to the richness of the lesson. For example, with no filter, our dark blue candle melted the most under the halogen light. However, using the sun & a magnifying glass, you can show that a magenta candle will melt the most. This is because the sun s light is most intense in the green portion of the spectrum (which again can be shown with a prism). As with any lab, the students have to perform it accurately in order to get valid results. Make sure that the candles are placed so that the focus is hitting them. Also, the focal point may change when a different color filter is used. Furthermore, the set up, timing & analysis need to be done in a consistent diligent manner. 35

37 Discussion Points: After each group has completed their lab, a whole class discussion can be very rich. In this discussion, it will be pointed out that the yellow & blue filters gave opposite results as did the green & the magenta filters. The reason why can be shown with a digital camera. If you make your digital camera view in a negative image, and hold up the yellow & blue filters, the students will see that the yellow filter is blue as a negative and the blue filter is yellow. The same will happen with the green & the magenta filters. Then you can show a color wheel that shows the primary (light) colors & their compliments. Then discuss that a green candle absorbs magenta light and reflects all other colors (it looks green because greens compliment is absorbed) etc. Thus you will establish through discussion that a colored candle will absorb light that is its complimentary color. Primary Colors & Their Compliments 36

38 Lab #3 Hot Erasers Introduction: Today your group will see how erasers of different color respond to focused light of various colors. As you proceed, make conjectures about what you see and discuss each others ideas. Materials needed: Small colored erasers Thermometer Laser Simulator Stop Watch Safety: Never look directly into the light source Do not place your hands at the focal point for more than 1 second when the light source is turned on. After every 30 minutes, turn the light source off for 5 minutes in order to cool down you can use this time to discuss your results Wear your sunglasses at all times when the light source is turned on Make sure the shield is in place when the light source is turned on Always be attentive to what you & your partners are doing 37

39 Procedure: Follow these steps every time that you test an eraser. Make sure light source is off Place the desired eraser on platform Turn on the light source Quickly adjust your eraser if needed to make sure that the ridge of the eraser is at the focal point Start timer Put up shield After the allotted time, turn off the light source Remove the shield Remove the eraser and then find its temperature. Analyze results I. Eraser Variation In this portion of the lab, you will compare what happens to the different erasers while you keep the filter the same color. For time sake, you will only test with 2 of the filters of your choice. A) Which two color filters did you choose? & 38

40 B) What color eraser do you expect to get the hottest with the filter? What color eraser do you expect to get the least hot? Explain your reasoning. C) What color eraser do you expect to get the hottest with the filter? What color eraser do you expect to get the least hot? Explain your reasoning. Place your first filter on top of the lens. Place the yellow eraser under the lens for 2 minutes and then record its temperature. Place the green eraser under the lens for 2 minutes and then record its temperature. Place the red eraser under the lens for 2 minutes and then record its temperature. Place the blue eraser under the lens for 2 minutes and then record its temperature. Place the purple eraser under the lens for 2 minutes and then record its temperature. Repeat these steps with your second filter 39

41 Filter Color: o Eraser Temp. ( F) Yellow Green Red Blue Purple Filter Color: o Eraser Temp. ( F) Yellow Green Red Blue Purple 40

42 D) Describe what happened. Did the erasers respond in the same way? How did your results compare to your predictions? List the erasers in order from hottest to least hot & offer an explanation as to why this happened. II. Filter Variation In this portion of the lab, you will compare what happens to one color of eraser while you vary the color of the filter used. For time sake, you will only test with two erasers of your choice. A) Which two colors did you choose for your erasers? & B) What color filter do you expect to make the eraser the hottest? What color filter do you expect to get the eraser the least hot? Explain your reasoning. 41

43 C) What color filter do you expect to make the eraser the hottest? What color filter do you expect to get the eraser the least hot? Explain your reasoning. Place the yellow filter on top of the lens. Place a fresh color #1 eraser under the lens for 2 minutes and then record its temperature. Place the magenta filter on top of the lens. Place a fresh color #1 eraser under the lens for 2 minutes and then record its temperature. Place the red filter on top of the lens. Place a fresh color #1 eraser under the lens for 2 minutes and then record its temperature. Place the green filter on top of the lens. Place a fresh color #1 eraser under the lens for 2 minutes and then record its temperature. Place the blue filter on top of the lens. Place a fresh color #1 eraser under the lens for 2 minutes and then record its temperature. Repeat these steps for eraser color #2. 42

44 Eraser Color: Filter Temp. ( o F) Yellow Magenta Red Green Blue Eraser Color: Filter Temp. ( o F) Yellow Magenta Red Green Blue D) Describe what happened. Did the filters heat your erasers in the same way? How did your results compare to your predictions? List the filters in order from the one which heated the each eraser the most to the one that heated it the least & offer an explanation as to why this happened. 43

45 III. Conclusions A) Do different colors of light effect objects in the same way? B) Try to generalize which colors of light effect which colors of erasers. Offer an explanation for your generalization. D) Compare your results with another group. 44

46 Teacher Guide Lab #3 Hot Erasers Purpose & Results: The purpose of this lab is to fully cement the ideas of color absorption by allowing students to make predictions of absorption based on the color wheel (discussed in last lab). The students should be able to predict that the magenta filter will heat up the green eraser the most or the yellow eraser would get heated up most by the blue filter & least by the yellow filter. They also will get good at predicting what will happen to the other colors involved. Technical Considerations: As before, the lab depends on the students, the filters, and the light source. The erasers need to be placed in the light and then their temperature measured in a consistent and reliable way. If your erasers are small, this may be hard to do. You should find a way that works best for your easers and thermometers and then demonstrate this to your students. Also, if the filters allow different amounts of light through, your results will be flawed. However, you can measure the light intensity for each filter with a photometer and then divide the temperature increase by the light intensity and compare these ratios. If your erasers are not good colors and do not exactly match your filters, your results may be muddled. I chose erasers because of their colors. If you cannot find good erasers, find another item that comes in a variety of colors that you can test. The trickiest 45

47 variable, however, is the light source. Again, if your light source does not give uniform light, your results will be confusing. However, if you know the spectrum (via a prism) of your light source, the students can take this into account and still predict accurately. For instance, once we knew that our light source gave off intense yellow light, we could predict that the blue eraser would always be one of the hottest erasers for each filter. Discussion Points: After the lab is complete, students should share their successes in predictions. They should get fairly good at this. However, discussions about surprises will be very rewarding as well. If you have an alternate light source with a different spectrum, you could show different results. After this discussion, the students should be primed for a short lecture on laser applications and how color choice is very important depending on the application. 46

48 Lab #4 Absorption Spectrum Introduction: Today you will investigate how a variety of materials absorb light at various wavelengths. As you proceed, make conjectures about what you see and discuss each others ideas. Materials needed: Variety of Objects Small Glass Containers Stop Watch Thermometer Laser Simulator Safety: Never look directly into the light source Do not place your hands at the focal point for more than 1 second when the light source is turned on. After every 30 minutes, turn the light source off for 5 minutes in order to cool down you can use this time to discuss your results Wear your sunglasses at all times when the light source is turned on Make sure the shield is in place when the light source is turned on Always be attentive to what you & your partners are doing 47

49 Procedure: Follow these steps every time that you test a candle. Make sure light source is off Place the desired object in holder Turn on the light source Quickly adjust your object if needed to make sure the candle is at the focal point Start timer Put up shield After the allotted time, turn off the light source Remove the shield Remove the object and measure its temperature Record results I. Collecting Data In this portion of the lab, you will choose 5 different objects and place them in the laser simulator under each filter. Place your first object under the laser simulator under your first filter Turn on the light for 90 seconds Remove the object and measure its temperature Record its temperature in the chart below Cool the object, rotate it, or replace it with an identical one Repeat process for all filters Repeat process for all objects 48

50 Temperature Under Each Filter for 90 Seconds Object Red Yellow Green Blue Purple II. Data Display Make 5 Temperature VS Color line graphs one for each object from your data. Summarize what your graphs tell you in the space below. 49

51 Teacher Guide Lab #4 Absorption Spectrum Purpose & Results: The purpose of this lab is to allow students to investigate how a variety of different materials absorb light at different wavelengths. Students should be able to make good guesses about the behavior of each material based on color. However, this also will depend on the specific heat of each substance. Although this method will not give an accurate atomic absorption spectrum, it shows students that materials absorb light differently. A follow-up lab using a spectrometer or a lecture on the atomic absorption spectrum will make much more sense after students have first completed this lab. Technical Considerations: As before, the lab depends on the students, the filters, and the light source. The materials need to be placed in the light and then their temperature measured in a consistent and reliable way. Also, if the filters allow different amounts of light through, or your light source does not give uniform light across the spectrum, your results will be flawed. However, you can measure the light intensity for each filter with a photometer and then divide the temperature increase by the light intensity and compare these ratios. It is fun for the students to have a variety of objects to try. Some interesting ones that we tried include meat, pop corn, hair gel, jell-o, coins, chocolate chips, gummy bears, small plastic Frisbees etc. 50

52 Discussion Points: After the lab is complete, students should share their results and display their graphs. Discuss surprises, including materials that got really hot & those that did not. The discussion can then lead you into a lecture/lesson/lab dealing with a variety of topics including specific heat, atomic absorption spectrums, spectrometers, emission spectrums, quantum theory, and laser applications. Also, the following demonstration would illuminate the ideas behind some new techniques in treating cancerous tumors with lasers. In these treatment, the patients ingest a solution that contains very small particles (nano- particles or quantum dots) of a substance with a known absorption spectrum (gold or silver etc.) These particles are enticed to attach themselves to the subjects antibodies which will attach to the cancerous tumors underneath the skin. These particles are chosen such that the wavelength of light that they absorb is a wavelength that human skin transmits. Thus, when a laser of this wavelength is shown on the skin above the tumor, the laser light passes through the skin and is absorbed by the tiny particles. These particles then get hot. Once hot enough, the tiny particles disintegrate the tumor and the tumor is removed without any invasive surgery. To illustrate how this could work, place yellow hair gel inside a glass container. Then take a white chocolate chip and place in inside the gel. Place the container I the laser simulator under yellow light and see how long it will take for the white chocolate chip to melt (if you can get it to melt). Then coat a white chocolate chip with metal (zinc works nicely) shavings and place this coated chip inside another glass container with 51

53 yellow hair gel. Place this container in the laser simulator under yellow light and see how long it takes the coated chip to melt. You can use thermometers as well to show that the surface of the gel does not get hot. The gel represents your unharmed skin and the melting chocolate chip the tumor, and thus you can show how lasers and absorption spectrums apply to the medical field. Furthermore, instead of using zinc shavings, you can stick small blue metallic decorations into the chocolate chip. This will demonstrate how the importance of the correct color choice of laser light in these types of applications. 52