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CoolStuff
Newsletter Article
Vol. 22, January 2006
Spectrum
Analysis: Breaking it all down. |
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The constituent colors in a beam of
light are revealed when the light is dispersed by a raindrop or
passed through optical instruments known as prisms and diffraction
gratings. In each case, an array of colors, or spectrum, is
observed.
Much of what we know about
the makeup of matter has been gained through spectroscopy, the
study of spectra. Our current understanding of atomic structure can
be traced back to Johann Balmer’s analysis of the spectrum of
hydrogen. On a larger scale, just about everything we know about
objects in the far reaches of the universe comes from examining
light. Since each element emits a unique collection of colors,
spectra, like fingerprints, may be used to determine the makeup and
physical condition of stars.
The colors contained in light from a
particular source are revealed when the light is scattered from an
optical instrument called a diffraction grating. Unlike a prism,
which relies on refraction to disperse different wavelengths of
light, a diffraction grating employs interference to separate
spectral colors. A grating consists of a very large number of
equally spaced parallel lines etched on the surface of either glass,
plastic or metal.
While most people are familiar with the beautiful array of colors
reflected from the surface of a compact disc, they may not realize
that, like a diffraction grating, a compact disc is covered with
thousands of tightly-spaced, nearly parallel lines. These lines are
sections of a spiraling line engraved on the disc’s surface. This
data spiral contains the information-bearing pits that are burned on
the disc during the recording process.
The availability and low cost of recordable compact discs make it
possible for every student in your class to make and use their own
spectroscope. In this edition of CoolStuff we provide instructions
for making three types of CD-R spectroscopes. The inexpensive
devices are easy to construct and will allow students to study the
spectra of light sources both in and outside the classroom.
Each type of spectroscope has unique advantages. The reflection
spectroscope allows students to observe both emission and absorption
spectra. Using this device they can observe the portions of the
white light spectrum absorbed by a variety of substances, including
chlorophyll. They may be surprised to learn that a leaf, the
universal embodiment of greenness, rejects green light and absorbs
light from the other portions of the visible spectrum.
Using the entire surface of a clear CD-R disc, the simple
transmission spectroscope described below has great light gathering
power. It may be used to study the spectra of sources of low
intensity such as LEDs and the moon.
The CD-R tube spectroscope resembles many commercially available
devices in both appearance and performance. The dramatic spectra
produced with this spectroscope are seen throughout this newsletter.
Any one of the three spectroscopes may be used in school or at home.
When used in your laboratory, we suggest that you set up as many
light sources for observation as possible. These sources may include
incandescent bulbs, fluorescent lamps, neon indicators, LEDs,
computer monitors…the list goes on. We have included a data sheet
that students may use to record their observations. If you have
students examine light sources outside the classroom, you may find
that they return with spectra of sources such as sodium and mercury
streetlights, neon signs, halogen headlights, and electric range
elements.
~Chris Chiaverina
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1. Reflection CD
Spectroscope
A CD reflection spectroscope
consists of a CD mounted diagonally at the end of a shoebox. The
figure below shows how the device is constructed. The CD may be held
in place with either a small piece of tape or by inserting it into a slit
cut in the bottom of the shoebox. A second narrow slit,
approximately 5-cm long and 5-mm wide, is cut in the shoe box lid.
Situated over the CD and parallel to the width of the shoebox, this
aperture admits the light to be analyzed. At the opposite end of the
CD a viewing window, roughly the size of a postage stamp, is cut in
the shoebox’s end panel. Trial and error is used to obtain the
optimal spectral display when positioning the CD.
Fluorescent Spectrum for Simon Quellen Field's CD
Spectroscope (See site link below for detailed instructions)
Unlike the other activities in this
issue of CoolStuff, any CD can be used to construct the Reflection
CD Spectroscope.
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Putting a plastic Petri dish over the
slit on the top of the shoebox permits the examination of absorption
spectra. When placed in the Petri dish, virtually any colored
solution will produce dark stripes on a continuous incandescent
spectrum. Solutions of chlorophyll, cobalt chloride, and potassium
permanganate are readily available and produce good results.
Students should never look at the Sun with these devices!
The plan for CD
Spectroscope is based on a design that appeared in an article by F.
Wakabayashi, K. Hamada and K. Sone, J.Chem.Educ. 75, 1569 (1998) |
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Actual photo taken using the instructions below; digital
camera, no room lights, no flash and incandescent light. To see the high
resolution image click here
2. Simple
Transmission Spectroscope
While observing the reflected spectra
from a CD is very revealing, a somewhat more convenient way of
performing spectral analysis is with a recordable CD (CD-R). A
CD-R without a label is used as a transmission grating. This type of
grating permits direct viewing of light sources.
When CD-Rs are purchased in packages of
30, 50, or 100, one or two clear blank discs may be included as a
form of protective packing material. While these discs don’t have
labels, they do have spiral data tracks and hence may be used as
diffraction gratings.
Students should never look at the Sun with these devices! |
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A clear CD-R may be
used as a grating simply by holding it about 20 cm from the eye and
centering the light source to be analyzed in the hole of the CD. A
particular advantage of this type of spectroscope is that it may be
used to study sources of low intensity such as an LED or the moon.
Students should never look at the Sun with these devices! |
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Removing Labels from CD-R
Discs |
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If clear CD-Rs with data tracks are not
available, you will have to remove the laminated label from an
ordinary CD-R. To do this, position the disc with the label side up.
Using packing tape or duck tape and a quick pull of one or two pieces of
the wide tape placed across the label will result in quick removal of the label. |
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3. CD-R Tube
Spectroscope
To cut the smaller discs that will be
used as spectroscope gratings, you will need a clear CD-R disc with
a data track, a cardboard mailing tube or paper towel roll, a pencil
or pen and a pair of tin snips or heavy duty scissors.
Repeat this process
to obtain additional gratings. You should be able to obtain at least
four gratings from a single CD-R.
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Place one end of the
mailing tube on the CD-R so that the edge of the tube touches the
outer edge of the disc (see figure below). Use a marking pen to
trace around the tube so as to leave a circular mark on the surface
of the disc.
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Use the tin snips
(or a large pair of good scissors) to cut around the circular mark
on the CD-R. |
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Now cover one end of the mailing tube
with an opaque material such as construction paper or aluminum foil.
A narrow slit, approximately 0.5 mm wide and 2 cm long should be cut
in this end cap to permit light into the tube. |
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The circular grating
is attached to the other end of the tube using household cement or
hot glue. Before attaching the grating, make sure that the grating’s
grooves are parallel to the slit. |
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The finished Spectroscope (above) and
resulting spectrum examples (below). |
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For more information on CD-R
transmission spectroscopes, see A Compact Disc Transmission
Spectroscope by Tim Knauer in The Physics Teacher magazine [Phys.
Teach. 40, 466 (2002)] and Compact Disc Spectroscopes Revisited! By
Aidan Byrne
[Phys. Teach. 41, 144 (2003)] |
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Here's a great tool for identifying elements when
viewing spectra.
Night Spectra Quest
The night is alive with lights whose
spectra are different, intriguing, and beautiful. Help your students
learn how to tell them apart by equipping them with this combination
holographic diffraction grating (spectroscope) and pocket-sized
spectra chart. If you simply hold the grating up to your eye, you
can view and identify mercury, metal halide, and sodium lights, as
well as incandescent, fluorescent, and neon. The principle behind
this spectroscope and chart is the same one that astronomers use to
discover what stars are made of.
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Click for details |
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Take it
Home...
by Diane Riendeau ~ Deerfield High School, IL |
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Why not have your students make
it, take it and then teach it? Have your students make a piece of
apparatus that illustrates a concept for each of your major science
units. Then have them take it home and teach parents and siblings
about what they are studying in class.
Making a piece of apparatus or creating a demonstration gives
students a sense of ownership. Students always take pride in what
they make themselves. Those who are not mathematically adept often
shine when allowed to apply their creativity to produce something
tangible.
As every teacher knows, you never really understand something until
you are asked to teach it. Encouraging students to explain phenomena
to others gives them an opportunity to make a concept their own.
The CD spectroscope has proven to be one of the most engaging
devices I've had my students construct. If you are interested in
increasing student understanding while getting families involved
with science, the CD spectroscope is for you! |
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Here's the link to download the student
worksheet for spectrum explorations.
WORKSHEET pdf |
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Think about plants...
Question for extra points: Why is the
absorption greater in the blue and red wavelengths?
Image courtesy of Ray at
www.firstrays.com
(online
Orchids store) |
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Interesting
Links:
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Next Issue:
In today's
schools, most subjects are presented in an insular manner.
Connections between disciplines are rarely discussed, much less
emphasized. Not surprisingly, students often fail to see
important interdisciplinary linkages. In the next issue of
CoolStuff we'll suggest activities that you may use to break
down the barriers between two seemingly disparate subjects: art
and science. As you will see, the intersection of the two
disciplines provides fertile ground for student learning.
Regards,
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