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
Here's how to build the spectrometer:
- 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.
- You will just hold your slide-mounted grating up to the open end of the
tube to view spectra. Ignore the sandwich grating construction in the
picture below: this is replaced by your slide, and is often referred to
below as 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. Which way should the slit be oriented if you want to see the
spectral details of the source?
- 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.
- 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.
- 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?
- Now look at different light sources and
compare what you see. Some suggested sources:
- an incandescent light source
- a fluorescent light source
- a colored LED (red, green)
- a white LED
- an orange-colored street light
- the sky (in whatever state of weather)
- a neon sign
- a candle flame (try slitless and stick something in to see orange sodium linein shape of flame if slitless)
- 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
- 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...