Building your own Pizza-Box Spectroscope Experimental Notes *You will need to bring in a medium- sized sturdy cardboard pizza box, shoe box, or similar from home. Color, Light, and Atomic Spectroscopy "Why are my socks red?" was the title of a popular science education poster that appeared in the Paris underground. The question "Why is the sky blue?" has puzzled both parents and many an inquisitive youth. In this lab we shall explore what gives rise to color and examine the emission spectra of atoms using spectroscopy. The interaction between light and matter can be used for a variety of tasks from determining chemical composition to measuring precise concentrations. We can use spectroscopy to determine what elements are present in the stars or what materials give rise to the colors we observe in everything from clothing to fireworks. In this experiment you will build and calibrate a pizza-box spectroscope and use it to explore the chemistry of color. Objective Your team s objective is to design a working pizza-box spectroscope. Calibrate it using a mercury lamp. Test your calibration using a hydrogen lamp. And then use your calibrated spectroscope to measure the emission spectrum from two other lamps and some flame-test salts provided. Finally you will compare your results to published spectra. Report For this experiment you will work in your teams. There is no special data sheet or report format for this experiment. The report consists of handing in a signed copy of your lab notebook data along with the answers to the seventeen questions posed in the experiment. The pizza-box spectroscope you construct is yours to keep! Only one report and copy of your data with all your team members names is required, but each team member should keep all the data and notes in their own notebook for future reference. Don t forget to get your notebooks signed before leaving the lab! 1
Building the Pizza-Box Spectroscope For this project you will need a cardboard box from home measuring at least 1.5 x 7 x 7 (4cm x 18cm x 18 cm). This is the size of a small pizza box. However, a slightly larger box such as a shoe box, medium pizza box, or other sturdy cardboard container of similar size will work just as well. Black electrical tape, clear plastic graph paper, a diffraction grating, safety knives, scissors, rulers, and black magic markers will be provided. Your instructor will show you how to construct the spectroscope. The basic design is shown below: Assembly notes Cut a hole a bit smaller than the piece of plastic diffraction grating material you have been given close to the edge at one end of your box (if you are using a rectangular-shaped box you should use the wider sides for the slit and grating). Make a thin slit about 5 millimeters wide opposite to this hole as shown above. Mount the grating loosely. Aim your spectroscope at a fluorescent lamp and look through the grating. You should see a spectrum (or rainbow) extending side-to-side (not top-tobottom) inside the box. If the spectrum appears top-to-bottom then rotate the grating by ninety degrees. Note the position of the spectrum you observe and mark it on the box so that you can cut out a cardboard flap accordingly (the exact position will depend upon your box size). Now fasten the grating securely in place by taping along its edges taking care that you can still see through its center. Cut a flap on three sides slightly wider than the area that you just marked to allow for mounting of the transparent graph paper scale (see diagram). Do NOT mount the transparent graph paper at this time. Finally, place a strip of black electrical tape on either side of the slit where the light enters your box. Use these two strips of tape to narrow the slit to a width of approximately 1 millimeter. Show your assembled spectroscope to your laboratory instructor before continuing. 2
Exploring your Spectroscope Now that you have built a working spectroscope it is time to explore the workings of your new instrument. In this way you can both determine the working limits of your spectroscope as well as potentially improve on its design! Record all data neatly in your laboratory notebook. Number and record your observations and the answers to these questions in your laboratory notebook and be sure to submit these with your final report. 1. How do you think the grating works? Explain in several sentences. 2. If you were to use your spectroscope to view a source that only emitted two wavelengths of light one red and one blue what would the spectrum you observed look like? Make a sketch showing the top view of your spectroscope and draw colored arrows to indicate path taken by the rays of red and blue light. Use solid lines to indicate real light rays and dashed or dotted lines to indicate virtual (or imaginary) rays. Are the lines that you observe real or virtual images? 3. Look up at the fluorescent lights in the room. Do you see a rainbow? What are the colors? Do you see any prominent lines? Does the spectrum appear different depending on whether you open the flap or not? Record all observations in your notebook. 4. Your laboratory instructor should have set up a mercury emission lamp. Using your spectroscope, observe the spectrum from the mercury lamp, how does it appear? Is there a rainbow or just bright lines? Why do you think the mercury lamp has a different spectrum from that of the fluorescent lights? Are any of the lines that you observe the same in both? Record all observations in your notebook. 5. Try moving the spectroscope closer and further from the mercury lamp. What happens to your spectra? Do the lines get brighter or sharper? Do the lines move? Is the position of the lines the same? What changes occur? Record all observations. 6. Now let s change the slit width. Vary the slit width by unpeeling one of the pieces of electrical tape and repositioning it on the box. How does making the slit larger and smaller effect the lines that you observe? What is the best width for your spectroscope? What did you base this determination on? Record all observations. Calibrating your Spectroscope All scientific instruments must be calibrated before any quantitative measurements are made. Commercial instruments are almost always calibrated by the manufacturer before they are actually used in the factory or laboratory. A transparent sheet of graph paper will be used to calibrate your spectroscope. With a scissor cut out a small section of transparent graph paper that is slightly larger than the flap you cut in the side of your box. 3
Use a permanent marker to label some of the lines on your transparent graph paper 1, 2, 3, etc. Write these numbers below the graph so that you can read them when the graph paper is mounted to the box. It does not matter whether you label each line 0, 1, 2, 3 or every 5 th line, or every 10 th line. As long as the spacing of your numbering scale is consistent, you will be able to use it to calibrate your spectroscope! Tape the transparent graph paper to your box so that you can see the spectrum on the scale. With the flap slightly open you should be able to read the numbers that you wrote while looking through the grating. Number and record your observations and answers to these questions in your notebook and be sure to include these with your final report. 7. Now aim your spectroscope at the mercury emission lamp again. You should see a deep violet or purple line (this may be hard to see), a bright blue-violet line, a bright green line, and a bright yellow line in that order. You may also see a faint split green line between the bright blue and green lines, and a faint red line at the far end of the spectrum. Record the color and position on your scale of each these lines in your notebook. (Remember you should always estimate one decimal place beyond the graduations marked on the scale of any instrument). You may want to adjust your slit width again at this point to get the sharpest lines possible while still maintaining a reasonably bright spectrum. Make notes about any changes you make to the spectroscope in your notebook. The exact wavelengths of the four most prominent lines are given below: Color dark violet (harder to see) bright blue-violet bright green bright yellow Wavelength 404.7 nm 435.8 nm 546.1 nm 579.0 nm 8. You now need to calibrate your instrument. This means that you need to devise a method of converting between the positions of the lines that you measured for mercury on your scale and their exact wavelengths given above in units of nanometers. Your team should determine the best procedure for doing this. You may want to consult with other teams about their solution. Part of your grade will be determined by how well you calibrate your instrument. You may want to leave some space in your notebook after this question to record any subsequent changes or refinements to your calibration method. Be sure to detail how you calibrated your instrument in your final report. 4
Testing Your Spectroscope Now that you have built and calibrated your spectroscope it is time to test how well your calibrated spectroscope performs! 9. Your laboratory instructor should have set up a hydrogen emission lamp. Use your calibrated spectroscope and this lamp to measure the visible emission lines of hydrogen. Be sure to record the color and position of each line in your notebook. Using your calibration method, convert the recorded position of these lines on your scale into wavelengths. Compare these wavelength values to the known wavelengths of the visible emission spectrum of hydrogen (these values can either be found in your textbook, online, or determined using the Bohr equation). Be sure to state or show how you obtained these values and properly reference any sources consulted. Calculate a percentage error for each line. If your average error for these lines exceeds about 3% you should either recalibrate your spectroscope (Step 8), or improve your calibration method until your results for the hydrogen emission spectrum are within this limit. Be sure to detail each step of your calibration refinement in your notebook and in your report. Do not proceed to Step 10 until the tolerance of your instrument meets these criteria. Measuring Spectra 10. Your instructor should have set up several additional emission lamps in the laboratory. Use your spectroscope to view the emission lines from any two elements other than mercury and hydrogen (e.g. Ne, Xe, Kr, Li, etc.). Do you see lines or bands? Record the colors and positions of the brightest lines in your notebook and sketch a picture of the spectrum you observe (you may want to use the blank visible spectra diagrams at the end of this document for this). Use colored pencils or denote the color of the lines below them in your sketch. If the lines you observe are too closely spaced to measure each one individually (or if they appear as brightly colored bands), then you may record, about 20-30 bright red lines between 9.8 and 10.2, or similar, or simply sketch what you observe. Repeat this for the second element. Be sure to label each spectrum with the names of the two elements you observe. 11. Wear your safety goggles during this step: Now let s look at the flame spectrum of some metal salts. Some of these salts are used to produce the spectacular colors observed in fireworks. Chemists can use flame colors to help determine which elements are present in a sample. Similar observations are used by astronomers to determine the composition of distant stars. Somewhere in your laboratory there should be a series of labeled salts in test tubes with accompanying metal wires and a Bunsen burner. Each wire should also be labeled with the formula of a salt and care should be taken not to mix these up or dip the wire from one sample into another so as to avoid contamination. Dip each metal wire into its salt sample and then place the end of the wire containing the salt into the center of the burner flame. Using just your eyes (not your spectroscope) observe the color of the flame made by each salt when it is placed into the flame using the wire. Record these observations in your notebook. 5
12. Wear your safety goggles during this step: Now we will use these flame-test samples and your spectroscope to measure the emission lines of just two of these samples. Using the sodium salt sample and any one other flame-test salt of your choice, repeat Step 11, but this time use your spectroscope to observe the emission lines given off when the salt is put into the flame with the wire. Record the color and position of the lines observed. You will need a friend to help hold the wire in the flame while you observe and record these data. It may take several trials to measure each observed line. Next, using your calibration method determine the wavelength(s) of the brightest line(s) for sodium and for the other element you selected. A CRC handbook should be available in the front of the room. You may want to use this handbook to begin Question 17 below by looking up the brightest lines in the spectrum of the two elements you just observed in the flame. Using the CRC to find spectral data can be tricky and it may be helpful to do this while your instructor is present. Have your instructor initial your notebooks before you leave. Additional Questions (to be completed at home or at the library) 13. Suppose you made your spectroscope from a giant pizza box several times bigger than the one you actually used. How would the brightness of the lines in the spectrum you observed change for the giant pizza box? How would the resolution (the ability to distinguish and measure two closely-spaced lines) of your spectroscope change? What else might change? Explain why each of these changes would occur. 14. Would the method you used to calibrate your spectroscope work for another group s spectroscope? Why or why not? 15. Suppose that you were to go into production and sell spectroscopes. How could you ensure that all the spectroscopes were calibrated? What aspects of the spectroscope would you need to be sure were always the same? What could be changed without affecting the calibration? 16. Suppose you were to observe the emission spectrum of a complex molecule instead of an atom. What would you predict the spectrum to look like? Justify your answer. 17. A CRC handbook is available at the reference desk in the campus library. Use this handbook (or another reputable online source) to look up the brightest lines in the spectrum of sodium and the other element you observed in the flame using your spectroscope. (Some guides to using these sources are presented on the next page.) Record the exact page number you found this data on in your notebook and the edition of the CRC handbook that you used, or properly reference any other source that you consulted. Do these lines match the ones you recorded? If so calculate the percent error for each wavelength that you measured. Calculate the percentage error for each of your observed lines. How good were your results? 6
Using the CRC handbook and other sources to look up spectral data Spectral data for the elements are typically given under, line spectra of the elements, persistent line spectra of the elements, atomic emission spectra, or similar headings. You should know the wavelengths of the lines you observed before you begin (convert your measurements to wavelengths using the calibration method you devised for your spectroscope). Start by finding the entry for the element that is the cation of the salt that you observed (in general anion emission lines do not appear in the visible region of the spectrum and so we shall not be concerned with these here). For example, suppose you examined BeCl 2 and observed a bright line at 528nm, in that case you would look up data for Beryllium (Be). You should check that the data you are consulting was measured in air and not in vacuum. There may be several listings for each element. Be sure to check the headings of these data carefully. If there is a choice, look for the persistent lines of the elements as these are the most readily observable to the eye. Wavelength data in the CRC handbook is given in units of Angstroms (Å), where 1Å = 1x10-10 m. Converting 528nm to Angstroms we get 5280 Å. Looking in the table at around that wavelength for Be we find the following data: Intensity Wavelength (Å) 80 5087.75 8 5218.12 20 5218.33 3 5255.86 64 5270.28 500 5270.81 20 5403.04 20 5410.21 5558. 10 6229.11 Search for the line with the greatest intensity closest to your measured wavelength. In this case we find a line at 5270.81 Å with an intensity of 500 for Be. Because this is the brightest line in this region of the spectrum it is probably the line we observed. Now we can convert 5270.81 Å into nanometers and use this as our theoretical value to calculate the percentage error in our measured wavelength. Spectral reference data for the persistent lines of the elements may also be found at several website including that of the National Institute of Standards and Technology (NIST) at: http://www.nist.gov/pml/data/handbook/index.cfm This website has instructions as to how to access these data. 7
Blank Visible Spectra This page has been included for your convenience. You may photocopy or duplicate this page for use in recording observations and answering questions in this experiment. in nm 400 450 500 550 600 650 700 750 violet blue green yellow orange red in nm 400 450 500 550 600 650 700 750 violet blue green yellow orange red in nm 400 450 500 550 600 650 700 750 violet blue green yellow orange red in nm 400 450 500 550 600 650 700 750 violet blue green yellow orange red 8