Teaching the spectra of visible light can be an engaging classroom activity. But, it’s always been challenging to find ways to go beyond simple passive demonstrations.
Hand-held diffraction “rainbow” foil (sometimes mounted in cardboard glasses) can be fun. But students often have difficulty even spotting the spectrum. “I can’t see it!” is the common complaint. And because each student observes their own “private” spectrum, it can be difficult to draw them into a discussion about what they’re seeing.
An exciting new way to augment, or even replace, rainbow foil activities is to display a live spectrum on a computer overhead projector. When you use the RSpec-Explorer video spectrometer system, everyone observes the same thing. Having a concrete example in front of the classroom makes teaching the material much easier. And you’ll find that a live video captures the students’ attention far more effectively than any other teaching aid we’ve used for this topic.
LED Array Spectra
Many educators prefer to begin by showing the spectra of individual colored LEDs, The LED colors are pretty – and, yes, that helps! (See accompanying video.) By starting out with familiar LEDs, we help the students connect what they’re seeing to their everyday experiences.
Using an LED array allows us to display multiple spectra simultaneously, clearly demonstrating that each individual wavelength is diffracted by a different amount. (See Figure 1) This novel presentation often has students leaning in as they contemplate something so attractive and so different from what they’ve seen before. There’s a lot of opportunity here for you to challenge your students to explain what they’re seeing.
The white LED at the top of the column (visible in Figure 1 and video) helps students see how the individual stacked colors are in a rainbow.
As the video that accompanies this article shows, we can easily transition from qualitative to quantitative observations using the graphing capability in the RSpec-Explorer system. When students see the intensity graph of each individual color, it helps them see how one moves from raw observations to scientific data.
We’ve found that the rapid changes of the live intensity graph as we interact with our light source transforms what was previously a monotonous visual experience into something that really grabs students’ attention.
Gas Tube Spectra
Traditional gas tubes are an easy next step after the LED array. Once again, a live video display rather than idiosyncratic hand-held “rainbow” foil assures that our students all see the phenomenon being discussed. The live intensity graph alongside the original spectrum is very effective. Students can easily see how the colorful qualitative raw data becomes much more meaningful when graphed. Figure 2 shows the real-time spectrum of a Hydrogen gas tube (red). The blue vertical lines are shown by the software to indicate where we would expect to find Hydrogen Balmer peaks. You can see the red data peaks match up with the blue.
We like to click a tool bar button in the software to freeze the Hydrogen spectrum graph. Then we swap the tube out for a Helium tube. Seeing the spectra of both gas tubes on the same x-y axis (Figure 3) helps make it clear that a spectrum is a chemical fingerprint, differing from element to element.
The video that accompanies this article shows how we can use a reference library to readily identify the contents of a “mystery gas tube.” This is a great demonstration for your class. Or, you can challenge your students in a hands-on lab to determine the contents of an unidentified gas tube. Of course, using spectroscopy to identify unknown objects is a cornerstone of astronomical research. You might want to tie this activity into an astronomical discussion or about the Curiosity rover that recently landed on Mars – both also use a the same “fingerprint” matching to identify “mystery” objects.
When is yellow not yellow?
The availability of a live video spectrum opens up the possibility of all sorts of labs on light and color. For example, the video that accompanies this article shows two yellow spectra: one of a lemon, and one of a yellow cell phone screen. The two “yellows” have very different spectra. The cell phone screen spectrum contains no yellow at all – just green and red. This is a wonderful illustration of how an RGB monitor can “trick” the human eye into seeing yellow.
Burning to do Flame Spectroscopy
As the accompanying video shows, you can also observe spectra of some common elements by burning them in a Bunsen burner. Using flame salts intended for this purpose, different elements will exhibit different emission lines. This activity is a bit more challenging than gas tube spectroscopy, but can be instructional and rewarding for your students. Seeing the spectrum in real-time makes the process easier. Students can record the live video. And they can capture bitmap/jpg images of the graph for inclusion in their lab reports. Students can also export XY graph points of the intensity graph to a text file for additional analysis at a later time.
The software that comes with the RSpec-Explorer system has advanced features that allow you load astronomical spectra. You can explore on-line Hubble data. Or, open the Mars rover ChemCam data in the system and study their spectra. Although this is a somewhat more advanced activity, it’s a natural one to follow up with after your students understand gas tube spectra.
A real-time video spectrometer makes it much easier to teach light and spectra. Your entire class can share the same real-time view of your gas tubes and other light sources. And, at the same time, they can see the resulting intensity graph. Students won’t have to struggle finding a spectrum in a tiny piece of plastic. And you’ll find it’s liberating to know everyone can see the examples you’re discussing. Walk up the screen or use a laser pointer to call out important points. Capture screens and include them in classroom handouts. Or, have your students operate the system in “identify the mystery tube” labs.
Field Tested Systems, LLC
Seattle, WA USA
Tom Field is the founder of Field Tested Systems, and is also a Contributing Editor at Sky & Telescope Magazine. He has been involved in the education field for the past two decades. With a passion for science education, Tom is has been on teams large and small that have developed hardware and software applications in use today by thousands of users on all seven continents.