Physics 8: Extra Credit
Build a Worthy Spectrometer
This assignment consists of building a spectrometer using the grating
loaned to you 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 will use it to do the tasks/experiments enumerated
beloweach worth some extra credit. If you do all portions, it will
be worth 2.5% of your grade. That's hot. But you have to
show me your spectrometer to get credit: I need to see that you
actually made it and that it works.
Allow several days to do this work, because you'll need access to both
night and daylight conditions.
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
- 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
- Once you have determined the correct orientation, tape the eyepiece to
the end of the paper towel tube. Your spectrometer is done (aside from
- 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 (i.e., distinct spectral lines) of the source?
- 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.
- 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.
- 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 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 spectral lines: less emphasis on strength of lines, which is hard
to judge. The plots should include:
- an incandescent light source
- a fluorescent light source
- an orange-colored street light
- the sky (in whatever state of weather)
- a neon sign
- Look the daylight sky and you'll notice narrow, dark absorption lines
(vertical if rainbow spectrum is left-right. These arise from elements in
the sun's atmosphere. Perhaps the easiest to see are the green and
blue-green lines from iron and hydrogen, respectively. Do these disappear
when you look at things on the ground (note, spectrum gets way
dimmer, but look carefully for the lines)? At clouds? Buildings? Glints
- You are bound to notice something interesting, unexpected, or
inexplicable during your exploration. Here is a place for you to relate
this. It could simply be the coolest thing you saw using your
spectrometer. Or it could relate to the construction, visual effects,
Now for another 0.5% extra credit (total of 3% available), write down the
sequence of 1's and 0's that emanate from your H-ITT transmitter when you
press the E button. Only write out the first burst, and leave out the 01
delimiters: only write down the 36 data bits. If your checksum ends up with
more than 8 digits, ignore all but the last 8: just leave the others out.
You will need to use your
own transmitter ID numberand not just the last three digits
(the number is found under the battery). If you don't have a
transmitter, use the last 6 digits of your student ID number. The
credit will break down like: 20% for getting the button code right; 40% for
getting the transmitter ID right; 40% for getting the checksum right.
Write down the ID number you are using in addition to the resulting binary
stream. It will be easier for me to grade if you also e-mail these two
numbers to me in the same form. Be careful: every bit counts!