Friday, May 15, 2015

Pizza Box Spectroscope

The pizza box spectroscope in action.
How many times can you say spectroscopy in one week?  When we make these awesome spectroscopes, I'm sure I come close to 100 times!  I love this fun spectroscopy project that I got from ChemEd13.  Ed Escudero led a hands-on workshop in Waterloo called "Make and take: construct an inexpensive and calibrated spectroscope."  I went home with my own pizza box spectroscope and a great project for the atomic theory unit.  I hope he'll do the same workshop at ChemEd15 for anyone who wants to give it a try.

The pizza box spectroscope is a great way to add a quantitative experiment to the classic atomic spectra observations that we all do in class.  With this project, my students can take accurate measurements of the wavelength of light and use their measurements to identify unknown elements.  Gone are the days of just looking at the pretty colors of the atomic spectra (which is fun too, don't get me wrong).  My students can take measurements that are as good as 1% accuracy from a pizza box!  You can read about constructing the pizza box spectroscopes here.

The pizza box spectroscope has a diffraction grating at the eye hole (closest to the eye), a slit to let in light on the opposite side of the box from the eye how, and a plastic rod running through the box.  The plastic rod is the critical piece of the spectroscope.  Each rod is scored so that it has one vertical notch on it.  The notch on the plastic rod can be aligned with a color in the atomic spectrum, which allows the student to determine the wavelength of the colored line.  The slit can be illuminated by shining a flashlight on the end of the plastic rod, to make the measurements easier and more accurate.  
Testing their viewing of the continuous spectrum.

We used the mercury lamp to calibrate the spectroscopes.  But before I turned on the mercury lamp, I had the lab groups test each other to make sure they were all seeing the same thing.  We opened a shade in the classroom and looked at the continuous spectrum from the sun.  One person in the group put the white mark on a color and passed the box to their partner without telling them what color they chose.  The other partner had to look through the spectroscope and identify the color the mark was pointing to in the box.  This little warm up exercise was very helpful to make sure everyone could see the colors in the spectroscopes and use the white mark on the plastic rod for measuring wavelenght.

The mercury lamp calibration.  You can see the rod, and the white dot from the slit pointing to the green line.
Next we used the mercury lamp to calibrate the pizza boxes.  The students used the known wavelengths of the three prominent lines to take the initial measurements.  Once they aligned the slit on the plastic rod to a color, the students measured the length of the rod sticking out the of the box.  With three data points, they plotted a calibration curve for their box.  There are two important points to mention about each box:  the students have to measure from the same side of the box each time, and they cannot switch plastic rods with another box.  Either of these changes will render their calibration curve useless.

Working in the dark to calibrate the spectroscopes.

I only have one spectrum tube lamp in my class, so we huddled around it to make measurements.  These pictures of the lights shining on the end of the plastic rods are so cool.  We spent about a week on this project total, including building the boxes. 

Students are measuring an unknown element with the pizza box spectroscopes.  Groups work together to view the spectrum and light the rod with a flashlight for each measurement.

The students were very proud of their pizza box spectroscopes.  I love the low-tech nature of this project because they can build their own instrument "from scratch".  I think this is the only chance they have all year to calibrate an instrument.  I usually calibrate the pH probes for them because it takes too much time and can be a little fussy.  Nothing else we use really needs to be calibrated.  This spectroscope project is a great way to show my students an important step in getting good data.
I can't get enough of this fun project.  

After the mercury tube, I put in the hydrogen tube and have them measure the prominent lines.  I could look at the hydrogen spectrum all day long; I love the colors.  This is when the kids get really excited by their percent error.  Most are less then 5% accurate!  (I get pretty excited about the results too, I won't lie to you about that!)
The hydrogen spectrum with a continuous spectrum "noise" in the background.

The cell phone flash lights are so very handy during spectroscopy week.  We end up working in the dark for several days because they are all working on different measurements at different paces.  With only one lamp, it limits the number of elements we can view at once, so the kids just adapt and work with their flashlight apps to measure the length of the rod as needed.

Students used their iPhone flash light apps to measure the length of the plastic rod.

The last phase of the project is identifying unknown elements.  I put several different tubes in for them to test.  They have to measure three lines from each to make their identification.  Of course, they also realize that each spectrum looks different, so the measurements become the confirmation of the identity, and helps them choose between the elements that look close to their observation.

Here's the helium spectrum with the slit aligned with the yellow line.
I have a set of R-Spec spectroscopes that we use for the astronomy class that I plan to add to the spectroscopy week next year.  After finishing with their own spectroscopes, I will give them a chance to collect data with the R-Spec cameras.  The lab work with the electronic equipment will be more rich with the foundation they get from building their own spectroscope.  I'm actually thinking about expanding spectroscopy week into a term elective for advanced science students who want to design their own experiments using spectroscopy techniques.

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