Black Holes to Blackboards: Seeing the Light
Sahuaro High School
What could be more basic to astronomy education than light?
Some say nature's colors are beyond human mimicking, that the artist's hand, no matter how skilled, cannot duplicate the hues of a simple rainbow. The universe, too, seems filled with objects that no artist could have ever conceived. Those Hubble Space Telescope images of the Eagle and Cat's Eye nebulae captured objects so striking that it's hard to believe they exist. But how do we teachers guide students from the ooohs and aaahs to a deeper understanding of color?
For openers, students can see light rays. Well, actually, they can't, unless you shake chalk dust into the air as you aim your trusty laser at the ceiling. Other simple tricks of this sort can reveal the nature of color. The invention of the holographic diffraction grating has made it easy to investigate the rainbow of the visible spectrum. One grating taped on an overhead projector or to the lens of a slide projector will give a bright, large continuous spectrum on a white screen. You need to pass the light through a slide made from a file folder with a quarter-inch slit cut in it.
Students can build their own spectrum projectors. It requires a cardboard box with a 200-watt light bulb and a couple of cardboard tubes with diffraction gratings and convex lenses attached. You can buy the kits from Learning Technologies or build your own with their gratings and plastic lenses from Edmund Scientific. Students can then investigate the action of filters and the addition of colors. They can also study emission spectra by replacing the 200-watt light with a hydrogen tube and making the room very dark.
The relationship between color and temperature is important for understanding stars. Students can project a spectrum onto a "colorometer," which is just a series of paraffin blocks with aluminum foil separating them. The contraption demonstrates that not all the colors in the light-bulb spectrum have the same intensity. The brighter yellow-green portion of the spectrum penetrates the paraffin to a far greater depth than the dimmer red or blue light. Students can draw a blackbody-like curve on a piece of overhead plastic when looking down on the blocks. If you attach a rheostat to the bulb and slowly dim it, students can watch the peak of the curve move toward the red. Students can even assign a temperature to their diagrams by inserting a thermometer into the box for a few minutes. It's a rough approximation, but works fairly well. The class can compare these diagrams to more conventional blackbody diagrams.
Of course, learning about the visible spectrum while ignoring the rest of the electromagnetic spectrum would be like learning about the Sun and then ignoring all the other stars. One good tool is a poster such as "The Milky Way at Every Wavelength." Another option is to buy slides of astronomical objects at different wavelengths to show students why astronomers use different bands of the spectrum to bring out specific characteristics in celestial objects.
The best demonstration I have seen of using different wavelengths to uncover different properties of objects is to have a local scientist bring an infrared camera in to your classroom. Don McCarthy of the Steward Observatory here in Tucson, who has done this for my classes many times, turns on a soldering iron and leaves it in the back of the classroom before he starts his demonstration. He then uses the camera to scan the entire classroom, asking students to identify what they are seeing. Students are always surprised to notice a bright, glowing object shining mysteriously from the back of the room, since the soldering iron gives off no visible light. In lieu of an infrared camera, you can try using an ordinary camcorder, whose CCD is sensitive to near-infrared light.
Once students see light in action, they can investigate its properties - such as frequency, wavelength, and interference - with wave experiments. Students create their own waves on a Wave Demonstrator (commonly known as a Slinky) or in a tank of water (old PSSC ripple tanks are still useful). Either springs or water waves can demonstrate reflection and refraction, while diffraction is best done with the water tank and a few wood blocks. A rubber hose bent into a parabola and laid half submerged in the water shows how incoming parallel waves arrive at a focal point and, conversely, how circular waves generated at the focus produce outgoing parallel waves. Playing with springs and water waves can give kids a three-dimensional sense for what waves are. Then teachers can discuss how velocity, wavelength, frequency, and energy are related mathematically.
The study of light should, after all, be hands-on. Once students understand the basic principles, they can go on to study the Sun, the H-R diagram [see "Stars Never Die, They Just Love Their Gas," May/June 1996, p. 8], and the beautiful images from Hubble and other observatories. These scientific instruments, cranking out voluminous amounts of data, provide breathtaking glimpses of the unseen universe by using different wavelengths of the electromagnetic spectrum. Each camera is like a different set of eyes linked to our original ones by radio transmitters and computers. Teachers of science should strive to give their students basic understanding about light and color so that they too will see the universe through different eyes, to perceive the intrinsic beauty and hidden truth in what they see.
JEFFREY F. LOCKWOOD is a high-school and college astronomy and physics teacher at Sahuaro High School and Pima Community College in Tucson, Ariz. His email address is firstname.lastname@example.org.
For more ideas about teaching light, see "There's More to Light Than Meets the Eye," The Universe in the Classroom, summer 1996, available at the ASP's web site.