Building a Spectrometer

Build a Worthy Spectrometer

This task consists of building a spectrometer using the grating handed out in class (come see me if you didn't get one). 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:

• The first action is to make a slit aperture at one end of the paper towel tube. This is where light will enter the spectrometer. Make the slit out of two pieces of stiff paper with very straight, clean edges defining the slit. Aim for a slit width of about 1 mm. Tape these in place at the end of the tube. The idea is to have the two straight edges parallel to each other with at least 1 cm of unobstructed slit length: don't worry if tape obstructs the ends of the slit.
• If you're fired up, you can try using razor blades to form the slit. These are perfectly straight and sharp, so you can get a very narrow slit and thus better spectral resolution.
• Mount your diffraction grating in a sandwich between two pieces of stiff paper each with little square windows about 1 cm across. Each of these (identical) pieces should be circular with a diameter matching that of your paper towel tube. If the piece of grating you have to fit between the two is too large, lop off the corners until it fits. This is your eyepiece.
• Before taping the eyepiece onto the paper towel tube, you need to determine the appropriate orientation. See tasks 1 & 2 below. In short, you need the long axis of the slit to be perpendicular to the dispersion direction.
• Once you have determined the correct orientation, tape the eyepiece to the end of the paper towel tube. Your spectrometer is done (aside from tweaking).

Suggested Tasks

1. Before taping on the eyepiece, 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. Record what you see/notice. Which way should the slit be oriented if you want to see the spectral details of the source?
2. Before affixing the eyepiece, 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 describe why we want to use a slit at all.
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 describe 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 graph what you see in the form of a spectral plot (running from about 400 nm to 700 nm, which is all you'll be able to see). Just be approximate, mostly noting where you see lines: less emphasis on strength of lines, which is hard to judge. The plots should include:
   1. an incandescent light source
   2. a fluorescent light source
   3. a orange-colored street light
   4. the sky (in whatever state of weather)
   5. a neon sign
6. Look the daylight sky and you'll notice narrow absorption lines. These arise from elements in the sun's atmosphere. 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. You are bound to notice something interesting, unexpected, or inexplicable during your exploration. Take some time to reflect on what you thought was the neatest thing you saw or noticed.

Back to Physics 10 Home Page