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This activity uses another aspect of spectra—spectral
lines. The key idea here is that every element has particular
frequencies associated with it. When a photon (a light particle) of
that particular energy hits the element's atom, the light is absorbed.
This creates dark bands in the spectrum at those frequencies. If there
is more than one element, these patterns are superimposed—and you can
tell the chemical compositions of the stars. You can use spectra in more terrestrial applications as well. The
dark lines occur when bright light shines through a relatively cool
cloud of gas. If you take that same element away from the bright
light, and heat it up, it will shine on its own—making bright
spectral lines at exactly the same frequencies as the dark lines in
the star. (Look at mercury- or sodium-vapor streetlights through a
grating or prism; you will see the lines.) You can use this to learn
the composition of an unknown substance. Heat it up and look at it
with a spectrograph. The bright lines will tell you the elements.
And why did scientists first decide to look at stellar spectra? One
important reason was to demonstrate that the stars were made of the same
elements that are present on the earth. Without spectra, we wouldn't know! Calculations about spectra and spectral lines should be left to
college. But even primary school students can look at the light through
prisms and gratings and see how light separates into its component
colors. Middle-grade students can look at vapor lamps through gratings and
notice how the pattern of bright, emission lines is the same when the
element is the same. They can also discuss the difference in
appearance between the spectra of incandescent bulbs and neon bulbs.
A number of words are associated with the term spectrum. A spectroscope (SPECK-truh-skope) is a device for looking
at a spectrum. So is a spectrograph (SPECK-truh-graf), but you don't
usually look through a spectrograph. Instead, you use it to make a spectrogram (SPECK-truh-gram).
This is a record of a spectrum—originally on film, but increasingly, in digital form. The whole endeavor is called spectroscopy (speck-TRAH-skuh-pee). If you do this kind of work, you are called a spectroscopist
(speck-TRAH-skuh-pist). The National Science Education Standards (1996) state that
"as a result of their activities in grade 9-12, all students
should develop an understanding of [how]...each kind of atom or
molecule can gain or lose energy only in particular discrete
amounts and thus be absorbed and emit light only at wavelengths
corresponding to those amounts. These wavelengths can be used to
identify the substance" (pp. 176, 180–181). Many Internet sites provide information about spectra.
Unfortunately, most are esoteric, intended for
practicing astronomers. These two, however, may be of
particular interest. Here's an interesting experiment that you may be able to do
(especially at home): It turns out that the shiny side of a CD or CD-ROM acts as a
reflection grating, that is, you can use it to see spectra! Find a CD
you dislike (or no longer need) and look at the reflections of light
sources (other than the sun) in the CD. Of course, you will see the
light itself. But off to the side (tilt the CD), you will see its
spectrum. Incandescent lights show a continuous spectrum—like a rainbow.
But some special lights, especially compact fluorescents, will show
an emission spectrum, that is, a spectrum made of bright lines
on a dark band instead of the dark lines in the bright band you have
been looking at. This emission spectrum is one kind of compact
fluorescent: You will not see the lines themselves, but you will see at least
three images of the compact fluorescent bulb, in the three colors of
the brightest line. You will see a similar effect with mercury- or sodium-vapor
streetlights. The spectra will be different, of course, since they
come from different elements.
This is a spectra page
on a web site devoted
to lighting.
Back to Stellar Spectra
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Isaac Newton—the English scientist and mathematician
who is so famous for discovering the principle of
gravity—should be just as famous for his investigations
into the nature of light. Think about his amazing discovery:
that white light is really made up of all colors at once.
And not only that, a piece of glass can be used to separate
the colors.
