CD Spectrum

This experiment uses a computer CD or DVD. If you have ever looked closely at either a music or computer CD, you have probably noticed that they produce rainbows. If you hold the CD with the shiny side up and let light from a lamp reflect off of it, you will see a very nice rainbow of colors. If you are using a regular incandescent bulb, you will see all of the colors of the rainbow: Red, orange, yellow, green, blue, indigo, violet. If you use other kinds of bulbs, you may find some colors missing.

To try this, you will need:

  • a CD
  • an incandescent light
  • a fluorescent light
  • a variety of appliances that have different colored indicator lights to show that
    they are on. You will find these on coffee pots, VCRs, etc.

First, try to darken the room. If possible, have the incandescent light be the only light source in the room. Position the CD so that you can see a bright spectrum (rainbow) and notice all of the colors. Now, turn on a fluorescent light and turn off the incandescent one. Again, look for the spectrum. You will probably notice a big difference. First, the rainbow will be much more spread out. This is because the incandescent bulb has a small filament (wire) in the center which glows white hot. This gives you a bright, concentrated light source. With the fluorescent bulb, all of the gas inside lights up, so the light source is spread out, giving a dimmer, fuzzy spectrum.

Depending on the kind of fluorescent bulb, you will also probably notice some colors missing. The one I used had almost no blue, indigo or violet, giving a spectrum made up of red, orange, yellow and green.

Try looking at the indicator light from a coffee pot with the CD. If the indicator is a neon lamp, you will see that the spectrum is made up of yellow and orange. Examine as many different kinds of light sources, to see which colors are found in the spectrum of each.

Scientists use the science of spectrometry (the study of the spectrum or rainbow) for everything from identifying unknown chemicals to determining the composition of stars.

Science Fair Thoughts:

Can you identify common substances, such as table salt (sodium chloride), by burning them and looking at the spectral pattern of the light from the flame? Do different brands of fluorescent bulbs have different spectral patterns?

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