Teacher resources and professional development across the curriculum

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Teacher's Lab
The Science of Light

IntroductionLight in ColorLaws of Light
 
 

what's important


spectrum imageIsaac 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.

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.

  • Look here for a history of astronomical spectroscopy. The "Color plots..." link is interesting and the source of some of the data used in this lab.

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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