Cereal Box Spectrometer
Purpose: To show students that with easily obtainable materials, they can make a
spectrometer, with which they can make better than qualitative, near instrument
observations of light sources, and learn about the “light fingerprint” and how it can be
used to identify elements.
Materials: Diffraction Grating paper sheets, which can be cut into 1”x1” or 2”x2”
squares for each group/student (can be obtained at
http://www.rainbowsymphonystore.com/difgratfilsh.html, or
http://scientificsonline.com/product.asp?pn=3054509&sid=2008FS&eid=2008FS&mr:tra
ckingCode=81388950-1C75-DD11-AFF6-
000423C27502&mr:referralID=NA&bhcd2=1220025793 ). Cereal Box. Cutting tool,
preferably a box cutter or X-acto knife. Cardboard. Construction paper. Masking or
Duct Tape. Light source (preferably gas lamps) Graph Paper. Possibly a meterstick.
Ruler.
Theory: A diffraction grating is made up of thousands of grooves cut into a transparent
surface. When light is incident on these slits, each slit acts as a point source of light. The
light from these sources interfere with each other, and these interference patterns produce
maxima and minima. There are specific places where these maxima are in phase, and
these areas are wavelength dependant. This leads a fixed location being a particular
wavelength, and you must only observe where the peak intensities are to determine the
wavelength of the light when the spectrometer is calibrated. As it happens, each element
has a different “signature” or “fingerprint,” so if you use a Krypton gas lamp you get a
different spectrum than Helium, even though the colors of the gas lamps are similar.
Information for the theory section obtained from:
http://gratings.newport.com/information/handbook/handbook.asp and
http://physics-animations.com/Physics/English/DG10/DG.htm
Method:
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Distribute materials. Each student/group needs 1 1”x1” square of
diffraction grating, 1 cereal box, 1 half sheet of construction paper,
access to tape, 1 piece of graph paper the same are as the top or bottom
of the cereal box.
Have students determine the optimal viewing angle of the diffraction
grating paper by having them start in front of a light source and the
student should be facing the light source looking through the
diffraction grating. Then, the student should move laterally, without
turning his or her head. When the diffraction pattern appears and is
center, have the student determine their lateral displacement, as well as
the distance to the lamp from the original position of the student.
Record these values.
The student should measure the distance from the edge of the box to
the center of the bottom of the box along the long side, and make a
mark here, in the center of the bottom. The student should then use this
value in a proportion with the two distances before as such
X
.
Distance from Original Position to lamp = .
Horizontal displacement
distance from edge of box
The student should now measure the distance “X” from the bottom
towards the top of the box and mark this point. (Our measurements
produced a view-port centered 9½ cm on the box top, and 20 cm on the
side for the slit position).
The student should cut a slot 1” to 1½” tall, the entire width of the side
panel, with this most recently marked point being the center of the
rectangle to be cut out.
6.
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Cut the construction paper into 2 strips that are the same width as the
panel of the box that now has a rectangular slot. Use the natural edge,
or fold the paper to make a good, straight edge. Tape one of these
pieces to the box so that the good edge is midway down the slot,
reducing the size of the slot by about half. Repeat the process with the
other piece of construction paper, but make sure to leave a very small
(less than 1 mm) slit between the 2 pieces of construction paper (See
Fig. 2).
Cut out a ¾”x¾” square in the bottom of the box around the first mark.
Place the diffraction grating over this square hole, and tape it in place
(NB: The diffraction grating is directional, so make sure it is in the
correct orientation before taping down.
Open the box top. Remove or tape back one long flap.
On the graph paper, mark a zero line on what will be the centerline.
Mark 5 lines away, 10 lines, etc.
Tape the graph paper to the other three flaps so that it is on the inside
of the box and the marked lines are facing the diffraction grating, and
tape these flaps together, so that the graph paper will be held in place.
To Use:
1.
Look through diffraction grating. Look directly at the slit, and line up the
slit with the light source.
2.
Rotate eyes to look down the box, at the graph paper, without moving the
box.
3.
If pattern is not completely visible, slowly and slightly move the
spectrometer laterally.
4.
Make and record observations about spectrum, and spacing of the spectral
lines.
Other Notes:
- The diffraction grating paper can be replaced with a CD-R which has been
stripped of its reflective label. This is more difficult to use, but may be more
easy to acquire, and makes it a completely “homemade” spectrometer.
- You can make a slot with a box “U” shaped piece of cardboard. This slot
would make it possible to move one of the pieces of construction paper. By
taping down the “U” but not the paper, and just sliding it up and down, you
can observe the effect of slit size on the patterns and measurements. (See
Fig.4).
- Original Idea for a cereal box spectrometer comes from:
http://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html and
http://www.exo.net/~pauld/activities/CDspectrometer/cdspectrometer.html
Limitations of this device:
- While this device is more quantitative than many other homemade
spectrometers, it is still exceedingly difficult to calibrate and make an actual
measurement with.
20 cm
9½ cm
Diagram 1: Position of View-port and Slit.
Graph paper
Bottom Flap
Side Flaps
Diagram 2: Position of Graph paper within the cereal box top. Slit is to the left side of
the image. On the graph paper line which will be over the center of the box-top, mark the
number 0. Count 5 lines in each direction, and mark each as 5. You may also choose to
mark the 10th lines.
Why Do We Care?
-We can use the spectrometer to determine the elemental composition of objects
and light sources as each element has its own unique fingerprint.
-We can use spectrometers to investigate color and see whether a color is purely
one specific wavelength, or a composition of multiple wavelengths, e.g. a green light may
come from a source producing a wavelength in the green part of the spectrum, or it may
be made up of two sources, like a source with a wavelength in the blue region and one in
the yellow region.
-Diffraction gratings are used in common everyday objects: CD’s and DVD’s.
Fig 1: Viewport. Diffraction Grating paper is in the square hole in the
Aluminum which is taped to the box.
Fig. 2: Side view of spectrometer. Viewport is to the left. Top flap of box top is
taped back, facilitating the viewing of the measurement markings on the graph paper.
Fig 3: Placement of graph paper at the end of the spectrometer. Graph paper is
taped to the three visible flaps, which are taped together.
Fig. 4: Demonstration of the actual size of the slit, as well as the functionality of
the cardboard slider “U” for the second sheet of construction paper, which is not fixed to
the box, apart from being under the slider.
Sample Spectra and their Sources
HELIUM
KRYPTON
MERCURY
NEON
OVERHEAD LIGHT