Pizza box spectroscope: Calibration (part 2)

Plastic rod coming out of a box.

The length of plastic rod sticking out of the pizza spectroscope is measured.

In Part 1, I asked “How many times can you say spectroscopy in one week?” When I make these awesome pizza box spectroscopes, I'm sure I come close to 100 times! In the previous article, I described Ed Escudero’s hands-on ChemEd 2013 workshop. Ed’s "Make and take” workshop gave me the opportunity to construct and take home an inexpensive and calibrated spectroscope. With Ed’s guidance I left with a quantitative experiment to the classic atomic spectra observations.

Student lining up a rod in a cardboard box.

Students lining up the spectral line with scratch on rod. 

In Part 1, I described how to make the pizza box spectroscope. This spectroscope has a diffraction grating at the eyehole, a slit to let in light on the opposite side of the box from the eyehole and a plastic rod running through the box — near the slit. The plastic rod is the critical piece of the spectroscope and makes measurements possible. Each rod is scored with one vertical notch; I refer to this notch as “the scratch”. By sliding the plastic rod left or right, the scratch can be aligned with a colored line in the atomic spectrum. Once calibrated, this allows the student to measure the wavelength of the light. The scratch can be illuminated by shining a flashlight on the end of the plastic rod to make the measurements easier and more accurate. With their calibrated instruments, my students can take measurements that are within 1% accuracy of known atomic spectra! Student can use their calibrated spectroscopes to identify unknown elements.

A calibration graph (wavelength of light (nm) versa length of rod outside the box (cm)) with three points and a negative slope -37.164 and an y-intercept of 1208.2.A calibration graph (wavelength of light (nm) versa length of rod outside the box (cm)) with three points and a positive slope -34.044 and an y-intercept of -21.266.Once the spectroscopes are made, students play a group warm- up “name-that-color” game to make sure students are able to line up the scratch with a color in a spectrum. To play this game, I opened a shade in the classroom so the students can see the continuous spectrum from the sun. One person in the group moved the scratch to a color in the spectrum. Their partner is challenged to identify the color without talking to each other. The other partner has to look through the spectroscope to identify the color their partner chose. The team is ready to calibrate their spectroscope when each member can identify their partner’s color twice in a row.

After warming up, we used the mercury lamp to calibrate the pizza boxes. Once students aligned the scratch with a color in the spectrum, they measured the length of the rod sticking out of the box. The students used the known wavelengths of the three prominent lines in the mercury atomic spectrum to plot a calibration curve for the box (Fig. 1). The students can measure the length of the plastic rod on either the right or left side of the box; the slope of the curve will be either positive or negative depending on their choice. Regardless of which side they choose, the students must measure from the same side of the box each time. With the spectroscopes calibrated, the students can use them to measure other atomic spectra, identify unknown elements or analyze a mixture of elements.

I shared this project at the December 5th meeting of the New England Association of Chemistry Teachers (NEACT). In the workshop we constructed and calibrated a pizza box spectroscope. After the calibration, we discussed possible roadblocks that would prevent the implementation of this project in school. It is this type of engaged peer discussion that makes conferences beneficial to both attendees and presenters. Several teachers raised the razor blades as an issue in their schools. The purpose of the razor blades is to provide a straight edge for the light slit that allows the sample light into the spectroscope. One of the participants substituted two carefully placed pieces of foil tape to create a 2 mm slit in place of the razor blades. Another teacher suggested using old gift cards taped to the outside of the box to make the light slit. Either of these modifications should eliminate the need for razors without compromising the quality of the data.

Another issue that was raised was the mercury lamp used to calibrate the boxes. Many schools have eliminated mercury in any form from their stock rooms — even the tiny sample enclosed in a glass emission spectrum tube. There are several ways around this problem. One possible solution — the one that Ed uses — is to use an old fluorescent light that actually contains mercury in the bulb for the calibration. This is a bit trickier for the students because it produces many lines with the mercury spectrum being most prominent. Another possible solution to the mercury lamp calibration problem is to use LED lights with known wavelengths for the calibration. This form of light won’t give a spectrum with multiple lines, but it will appear as a single colored spot in the spectroscope that students can point to with the scratch. You need to have three different LEDs to make a good calibration. A third possible solution to the mercury lamp problem is to use another element such as hydrogen to calibrate the spectroscopes and avoid mercury altogether.

People holding cardboard boxes with protruding rods.

NEACT workshop. December 2015. Each participant made a pizza box spectroscope to take back to their classrooms.

The third predicted hurdle was access to the atomic emission spectra apparatus. Many schools do not own the spectrum tube power supply or the sealed gas tubes to use in this experiment. Although my school has one, it would be helpful to have additional lamps for students to view. The first solution proposed was to borrow the equipment from a local college or university. Many colleges are willing to loan out equipment to science teachers in their area, and may even have several lamps and a variety of samples. A member of an AP Chemistry teacher Support Network in Massachusetts had an immediate response to the lack of equipment; through the network she would try to find another chemistry teacher who has the equipment and is willing to share. A third solution was to appeal to grant money or department funds that are available in their district to purchase a classroom set of atomic emission spectra apparatus — without the mercury tube.

In the spirit of the ChemEd Conference, all of the participants in my NEACT workshop left with their own spectroscopes to use in class and with ideas for adapting the project to their unique teaching environment. A morning well spent!