Building a Spectrometer

Build a Worthy Spectrometer

This task consists of building a spectrometer using the grating handed out in class. You will need a paper towel tube, tape, and some stiff paper or thin cardboard. For the latter, you can use a cut-up cereal box, poster board (darker is better), or anything else you can get your hands on. Once you have built the spectrometer, you can use it to do the tasks/experiments enumerated below.

Here's how to build the spectrometer:

Familiarizing Tasks

  1. Hold the eyepiece to your eye and orient it so that the dispersion is left-right (look at a bright light and see that the associated rainbow is to the left and right of the central image). Now hold the tube (with slit attached) pressed up to the eyepiece, so that you are looking through the slit at the far end of the tube. Point the spectrometer at a fluorescent light so that you can see bright spectra to the left (or right) of the slit— along the inside of the tube. Rotate the tube along its axis while holding the eyepiece fixed. Which way should the slit be oriented if you want to see the spectral details of the source?
  2. Go out at night and look at a street light (an orange or bluish kind) just through the eyepiece. You'll see several "images" of the light in different, distinct colors, but some may overlap. You want to be close enough to recognize the shape of the light, but not so close that the images totally overlap and become confusing. After gaining more experience using the spectrometer with a slit, come back to this and muse on why we want to use a slit at all. A bare CFL light bulb can also serve the purpose of this exercise.
  3. In the daytime, look at the sky through a screen or other fine mesh. What is different about the view (compared to without the screen), and how do you interpret what's going on? To guide your thinking, you may also hold a paper clip (or any thin stick) in front of the slit and see if you can figure out how it affects things. If you don't have access to a screen, use the paper clip and imagine what you think you'd see if looking through a screen.
  4. Look at a fluorescent light through the spectrometer. If you change the width of the slit, how does it affect the appearance of the lines? If you want the best resolution you can get, what must you do to the slit?
  5. Now look at different light sources and compare what you see. Some suggested sources:
    1. an incandescent light source
    2. a fluorescent light source
    3. a colored LED (red, green)
    4. a white LED
    5. an orange-colored street light
    6. the sky (in whatever state of weather)
    7. a neon sign
    8. a candle flame (try slitless and stick something in to see orange sodium line—in shape of flame if slitless)
  6. Look the daylight sky and you'll notice narrow absorption lines. These are the Fraunhoffer lines of the solar spectrum, simply reflected by the sky. Do these disappear when you look at things on the ground? At clouds? Buildings? The prominent absorption line in the orange region is from sodium. This is actually a double line. Can you see it as double, or does your spectrometer lack the resolution?
  7. If you try real hard, you may be able to see the Calcium H and K lines at 397 and 393 nm. This is right at the edge of human visual perception, so the source needs to be very bright. Don't look directly at the sun, but you may benefit from a bright reflection off of glass or the like. Use a very narrow slit for good resolution and brightness suppression. Don't do this one unless you are so curious (mildly insane) that you don't mind risking some after-imaging in your eye. You're responsible for your own eyes here. That said, I've done it and thought it was cool...

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