 Angela Turricchia
Laboratorio per la Didattica Aula Planetario
Comune di Bologna, Włochy
 Ariel Majcher
Center for Theoretical Physics, PAS
Warsaw, Poland
A home-made spectroscope helps to realize that different sources of light do
not shine in the same way.
A spectroscope decomposes light that
comes to you into components in a form of spectrum with use of diffraction
grating. An
example of spectrum can be seen below.
You can build your spectroscope in two versions: the basic one, which can be
constructed from elements around you and the "lux" one, which is also easy to
make, yet requires custom-made elements.
1. Casing:
- basic version: a cardboard tube from inside of the roll paper
towels, a piece of a thin cardboard, e.g. from an empty tea or rice box etc;
- "lux" version: you can use a long cardboard tube used for
carrying papers. In general, the longer is the tube, the better is the angular
resolution of the spectroscope, but also the more light has to fall on the slit
i.e. one has to look at brighter sources.
Apart from that you will need opaque adhesive
tape (I used insulating tape). It is comfortable to use two-side adhesive tape
in order to make a slit, though it is not absolutely necessary.
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| Fig. 1: a tube for carrying pictures and other
elements necessary for making a spectroscope
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2. Slit:
- in
the basic version: from
a cardboard box,
- in
the "lux" version :
from unused razor blade (pay attention to safety of children!).
3. Diffraction grating:
-
basic
version: we made a
real discovery here. A good diffraction grading can be made from plastic disks,
often used to protect the last, bottom CD in CD bundles that are bought in
shops. We are talking about bulk packs of CDs, which are available in supermarkets.
Warning: some CD packs do not contain such additional protection disks, and not
all of them are suitable. A proper protection disk shall have a rainbow
glitter, when we look at it against the light. So you must watch what you buy.
One can
obtain a similar grating from a CD disc. One should scratch a bit the
cover on an edge of the disc, put a piece of a scotch tape over the scratch and
carefully remove the tape. The
paint should stay on the tape. Repeating this procedure a few times one can clean
the whole surface of the disc from the protective paint, obtaining a very good
diffraction grating, see figures below:
Scratch a bit of paint from the edge
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Put a tape on the scratch
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Remove the tape with paint
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Ready !!!
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Fig.3: turning a CD into a diffraction grating.
-
“lux”
version: a real diffraction grating can be bought somewhere. In Poland we buy them at Zamkor Publisher (www.zamkor.pl). You will need a grading of
resolution of 500 lines/mm.
4. Tools:
solid scissors, the best would be utility
scissors, a sharp knife (e.g. a special knife for cutting paper), a pencil, a
ruler.
Making of a spectroscope
Casing:
on a cardboard from a box we draw with a pencil
two circles in shape of the both endings of a roll from paper towels. It is
enough to press the both roll endings to the cardboard and to outline them.
Then, we cut the circles leaving a bit of additional space from each side. They
will serve us as lids:
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| Fig. 3: lids cut out of cardboard, we can see a
shape of razor blade on the right one |
Slit:
- basic version:
we will need one more cardboard circle. We should cut it into halves with one
move of scissors. It is also recommended to cut outer edges. It will allow us
to form a slit in a simpler and more precise way. It will be easier to stick it
to the frame. In the second circle we should cut a rectangular in the middle
(not too wide, but it should have about 80% of the circle's length) and stick
the both halves of the first circle to the other one in such a manner to form a
narrow slit with parallel sides (two-side adhesive tape will be a great help
here). In my spectroscope a width of slit is a bit smaller than 1 mm, but feel
free to experiment. Narrower slits require stronger sources of light, whereas
wide slits do not allow to observe solar spectrum. In the end we should
carefully cover the lid with an opaque tape, so that it did not allow light
except for slit.
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| Fig. 4: elements of slit and
ready-made cardboard slit |
- "lux" version: we should act in the same way. The only
difference is that in case of slit we should use a razor blade instead of
cardboard. We must cut the razor blade along the longer side into two more or
less equal parts and then stick it to the lid with a cut out rectangular. Its
blades should form a narrow slit:
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| Fig. 5: elements of slit and ready-made razor
blade slit |
Diffraction grating
In the second cardboard lid we should cut with
a knife a bigger rectangular hole. With scissors we shall cut out of a plastic
disc a rectangular bigger than the hole, but smaller than the lid's area. It
will work as our diffraction grading. We stick it to a circle with cut out hole
in such a manner to make it completely cover the lid. The final step, just like
before, is to cover the diffraction grading with opaque tape, to make it
impervious to sun, except for the rectangular in the middle.
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| Fig. 6: elements of
diffraction grating and ready-made diffraction grating
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Assembly
We shall stick the lid and diffraction grading
to one end of a tube from paper towels with use of a tape, remembering to
ensure light-proofness in joint places. If we do not see to it, noticing of
spectrum can become very difficult, especially when we look in direction of the
Sun. The slit should be placed at the other end of the roll and we should look
at a bulb from the side of diffraction grading as if through a telescope. If we
look at a small angle, that is "above" or "below" the source of light, we
should see spectrum. Next we should twist the lid with slit, which enables to
set position in which direction of lines of diffraction grading is parallel to slit
- in such orientation spectrum is visible most clearly:
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| Fig.7: incorrect and correct setting of
diffraction grid with respect to the slit
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Now we should fix with tape the lid in that
position and our spectroscope is already in working condition:
Fig. 8: appearance of a ready-made spectroscope
The execution time, after preparing all
essential materials, does not exceed half an hour.
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Now we can watch various sources of light. If
the source of light is strong enough - we will see a second row of spectrum,
and if, instead of naked eye, we will look through a camera (it would be ideal
if both a spectrograph and a camera were on a tripod stand), our toy
spectrograph will change into a real spectrograph and it will be possible to
register various spectra on a plate or CCD matrix.
Sample spectra:
classical bulbs have continuous spectrum (here a
characteristic of digital image becomes visible, to the naked eye a view is a
bit different):
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Fig. 9: spectrum
of a classical bulb
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the same applies to halogen bulbs:
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| Fig. 10: halogen bulb and its spectrum
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energy-saving bulbs have clear spectrum lines:
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| Fig. 11: spectrum of an energy-saving bulb
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as do modern fluorescent lamps:
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| Rys. 12: spectrum of a modern, fluorescent lamp |
old-fashioned fluorescent lamps also have spectral lines, visible on a continuous spectrum background:
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Fig. 13: spectrum of an old-fashoned fluorescent lamp
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the same applies to street sodium-vapour lamps:
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| Rys. 13: spectrum of a street sodium-vapour lamp
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and mercury-vapour lamps:
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| Rys. 15: spectrum of a mercury-vapour lamp |
If you act with due caution, you can also look
at the Sun - due to the fact that spectrum is visible at certain angle from the
source, so you never look directly at the source. Besides, we look through a
narrow slit, which reduces amount of coming light. Be cautious - looking in direction of the Sun
can always be dangerous to your eye! Fraunhofer lines are clearly visible in a
lux-version of the spectroscope.
Rys.16: our spectroscope and a camera prepared to take image of Fraunhofer lines
If you take precautions to prevent your
spectroscope from burning, you can try to see spectrum of earth gas in a gas
cooker, candle or a bonfire at a holiday trip.
Have fun!!
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Download instruction (pdf)
