The Spectrum |
If you have the chance to use a real spectroscope through a normal laboratory experience, please do so. However, if such an experience is not available to you, you can produce your own spectra without any expensive equipment at all.
Today, most spectroscopes and spectrographs no longer use prisms. Instead, they employ a small piece of glass with thousands of fine lines etched in parallel across its surface. This is called a "diffraction grating." It also serves to spread white light out and also produces a spectrum. Although true diffraction gratings are very expensive, inexpensive acetate replicas also are available that do an acceptable job for demonstrations. Still, you probably don't have one, do you?
Well, wait a minute, maybe you have something almost as good! Find an old CD -- maybe that group you can no longer stand -- and take a good look.
The data or music on the CD is stored in a series of concentric bands (really it is just one tightly spiralling band) that is similar in many ways to the grooves in a diffraction grating. In fact, it works similarly when reflecting light. Cut a thin slit, perhaps 1/16th of an inch wide and one to two inches long, into a piece of black construction paper or opaque cardboard. Then try shining a light --such as from a flashlight-- through it onto a light colored wall in a darkened room. You may need to play with it a bit to get it to work properly, but you should see a rainbow like spectrum on the wall. (In fact, a real rainbow is nothing more than a spectrum produced by billions of tiny water droplets, which act like prisms, on sunlight.)
You should see a continuous form of the spectrum because the light from a flashlight and most other light sources is from a heated solid filament. (Fluorescent lights produce an emission spectrum, but you cannot detect that in this observation for a couple of reasons. First, fluorescent lights usually are not focusable in the same way a flashlight is, so their diffuse light lights up the room too much. And secondly, fluorescent lights usually have many closely spaced emission lines, such that they may look pretty much like a continuous spectrum. This set-up does not provide enough resolution to see the individual emission lines.)
Try several different light sources if you can. If you have a small laser pointer, try that, too, but be careful not the shine it in anyone's eyes. What kind of spectrum does the laser pointer produce? Why?
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