 Maite Lacarra
Université Pierre et Marie Curie
Paris, France
 Ariel Majcher
Center for Theoretical Physics, PAS
Warsaw, Poland
Spectra around us
- studies with a home-made spectroscope
Have you already got your
own spectroscope? If not, build a spectroscope following the instruction from here
and point it at different
sources of light. You will see that not every source of light shines in the
same way. A spectroscope decomposes
light that comes to you into components in a form of spectrum with use of diffraction grating.
Two examples of such spectrum can be seen below:
We suggest here three
exercises consisting in examining spectra of different sources of light, which
can be done with use of your spectroscope. To do the exercise you will need a
sheet of paper and something to write with, in order to put down the results of
your observations. The spectra can also be photographed, but for the purpose of
this exercise, a sheet of paper, a pen and possibly coloured felt-tips shall be
just fine.
- 1.
Observation of spectra of
different sources of light and comparison of observation results with provided
examples. This allows to identify different types of light bulbs - classical,
energy-saving, fluorescent lamp.
- 2. Identification of the
observed spectrum lines in emission spectra.
- 3.
Observation of a yellow
spectrum line of sodium.
- 4. Observation and
identification of absorption lines in a spectrum of sunlight.
Part I: Observation of spectra of different light sources.
Point your spectroscope at
different sources of light in your environment: light bulbs in your flat,
street lamps, neon signs advertising shops and goods. Draw each observed
spectrum and compare it to the spectra presented below. Does any spectrum match
an exemplary one? If so, which of the examples?
Let's start with a
continuous spectrum. This is a source, whose light is composed of all
visible colours:
Below you will find
photographs of spectra of different sources of light made with use of a
home-made spectroscope and a digital camera. Your task is to compare spectra
seen by you after pointing your spectroscope at light bulbs and lamps in your
neighbourhood, with provided examples. For that purpose, it is recommended to
print an image of a continuous spectrum and then draw on it a position of the
observed spectrum lines.
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| spectrum 1 |
| |
 |
| spectrum 2 |
| |
 |
| spectrum 3 |
| |
 |
| spectrum 4 |
Spectrum no. 1 - a
classical bulb or a halogen bulb.
Spectrum no. 2 - a
fluorescent lamp of the old type.
Spectrum no. 3 - a
fluorescent lamp of the new type. It has a richer spectrum and its light is
more like sunlight.
Spectrum no. 4 - an
energy-saving bulb.
Discuss conclusions drawn
from your observations with your classmates.
Have you succeeded, basing
on the spectrum, in defining what sources of light can be found in your
neighbourhood? If you find a lamp whose spectrum differs from the ones
presented in this exercise, you can search spectra of different lamps in the
Internet.
Part II: Yellow spectrum line of Sodium
Thanks to observation of
different sources of light by a spectroscope, we have learnt that light from
some sources is not of a continuous character, but it is composed of clear
spectrum lines. Spectra of emitted light are a kind of IDs of elements and
particles. Each element and each particle has its own characteristic spectrum.
Spectra are divided into emission ones, created when light is emitted by
elements, and absorption ones, associated with absorption of light.
If you find the spectra
of elements interesting, we recommend you to visit Java animation illustrating spectra of
elements.
Apart from a
spectroscope, you will need a candle and some salt to do this exercise.
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| Fig. 1: A spectroscope with a
camera, prepared to photograph a spectrum of a candle
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Try to find a bit burnt-out
candle, in which there is a small depression in a stearin around the wick. Pour
some salt around the wick and light your candle. The experiment shall be
conducted in a darkened room or in the evening, in order to reduce the
background impact, as the flame of a candle is not very bright. In a
spectroscope you will see a clear, yellow line. This is a famous sodium
doublet, two closely located lines (resolving power of our home-made
spectroscope does not allow to separate them), which comes from sodium (Na)
emission:
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| Fig. 2: A spectrum of a candle
with a salted wax. An orange sodium line is clearly visible.
|
Think about a chemical
formula of a table salt. Does it contain sodium?
Warning! Be extremely
careful not to bring about fire!
Part III: Emission Spectra - Identification of Spectral Lines.
Identification of stellar
spectrum lines is not that simple, that is why we will start from an easier
task. Your task in this part is to identify lines observed in spectra of glow
tubes. Identification signifies defining what wavelength they match and
ascribing emission coming from a given element or particle to each line, on the
basis of information included in the exercise.We will start by
reminding of a continuous spectrum in comparison with a precise scale. It is
the base that allows to estimate wavelengths of light corresponding to
particular spectrum lines:
 |
Fig. 3: Visible continuous
spectrum
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Print an image of a
continuous spectrum and draw on it location of spectrum lines in spectra of
lamps observed with your spectroscope. Below you will find some information on
wavelength corresponding to emission lines in spectra of different lamps.
Details of spectra of
different bulbs (source: Wikipedia):
1.
spectrum of a fluorescent
lamp of the old type:
 |
| Fig. 4: Intensity of spectrum
lines in wavelength function in a fluorescent lamp of the old type
|
| Line number
|
Wavelength (nm)
|
Element
|
| 1 |
364.24 |
mercury
|
| 2 |
403.53 |
mercury
|
| 3 |
434.83 |
mercury
|
| 4 |
545.63 |
mercury |
| 5 |
576.35 |
probably mercury |
| 6 |
578.15 |
probably mercury |
Table 1: Identification of peaks
in a spectrum of a fluorescent lamp of the old type
As you can see, it is a
simple spectrum, so you should have no problems to identify spectrum lines.
Moreover, all spectrum lines in visible light come from one element - mercury.
2. Spectrum of a modern
fluorescent lamp:
 |
| Fig. 5: Intensity of spectrum
lines in wavelength function in a modern fluorescent lamp
|
click here to enlarge
Spectra of modern
fluorescent lamps are much more complicated, because they shine with light
similar to sunlight. Such spectrum has more lines, which makes an
identification more difficult.
Line number
| Wavelength [nm] | element
|
| 1 |
405.4 |
mercury
|
| 2 |
436.6 |
mercury |
| 3 |
487.7 |
terbium Tb3+ |
| 4 |
542.4 |
terbium Tb3+ |
| 5 |
546.5 |
mercury
|
| 6 |
577.7 |
probably terbium Tb3+ or mercury
|
|
| 7 |
580.2 |
mercury or terbium Tb3+ |
|
| 8 |
584.0 |
probably terbium Tb3+ or europium Eu+3:Y2O3 |
|
| 9 |
587.6 |
probably europium Eu+3:Y2O3 |
|
| 10 |
593.4 |
probably europium Eu+3:Y2O3 |
|
| 11 |
599.7 |
probably europium Eu+3:Y2O3 |
|
| 12 |
611.6 |
europium Eu+3:Y2O3 |
|
| 13 |
625.7 |
probably terb Tb3+ |
|
| 14 |
631.1 |
probably europium Eu+3:Y2O3 |
|
| 15 |
650.8 |
probably europium Eu+3:Y2O3 |
|
| 16 |
662.6 |
probably europium Eu+3:Y2O3 |
|
| 17 |
687.7 |
probably europium Eu+3:Y2O3 |
|
| 18 |
693.7 |
probably europium Eu+3:Y2O3 |
|
| 19 |
707 et 709 |
probably europium Eu+3:Y2O3 |
|
| 20 |
712.3 |
prawdopodobnie europium Eu+3:Y2O3 |
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| 21 |
760.0 |
probably argon Ar |
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| 22 |
811.0 |
probably argon Ar |
|
Table 2: Identification of
elements in a spectrum of a modern fluorescent lamp
Having all that data we can
start our exercise:
I.
compare spectrum observed
by you with a picture of a continuous spectrum (Fig. 1) and check as
precisely as possible which wavelength corresponds to each spectrum line.
II.
try to identify the
spectrum, that is to ascribe it to a particular type of a lamp. To achieve it,
you have to compare location of the observed spectrum lines with location of
spectrum lines of different lamps, by means of determined wavelengths of
different spectrum lines. Beware of different scales in different figures!
III.
look at the tables below
the images of spectra and find elements whose emission is responsible for the
particular lines;
IV. put down the results of
your observations in a table:
Lamp type
| Line description
| Wavelength
| Element | Distinctive features |
| eg. halogen bulb |
e.g. group of blue lines,
poorly visible
|
|
|
|
V.
when you have completed
your exercise, compare your results with your classmates.
Part IV: Observations and Identification of Absorbtion Lines in a spectrum of sunlight - Fraunhofer lines.
To observe a spectrum of
sunlight, you must build a spectroscope with a long tube. The longer the tube
of a spectroscope, the better resolving power and the less significant
influence of imprecision resulting from slit making. This exercise is based on
observations with a spectroscope made of a 1-meter tube. Such tubes are easily
accessible in the Internet and are quite cheap.
WARNING!!! Under no
circumstances you should look at the Sun, because you can damage your eyesight.
It is not necessary to look
directly at our daily star, in order to observe its spectrum. If in a nice,
sunny day we point our spectroscope at a clear sky, we shall observe a spectrum
of a diffused sunlight. On the continuous spectrum we should clearly see
several dark lines. They are absorption lines coming from absorption of light
by elements occurring in external layers of the Sun's atmosphere. They are
known as Fraunhofer lines, because that is the name of one of their
discoverers.
Below you can see a set
prepared to photograph a spectrum of sunlight:
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| Fig. 6: Spectroscope with a
camera prepared to photograph a spectrum of sunlight
|
It is recommended to cover
your eyes/camera with some diaphragm, like the one visible in the photo, a
sleeve made from a black cardboard fixed to a spectroscope. It will enable to
enhance contrast and will improve spectrum visibility.
This photograph presents a
spectrum of sunlight with Fraunhofer lines:
Unfortunately, each
digital camera has built-in filters (which results in an image of colour) and
the quality of such photograph is much worse than the image visible to the
naked eye. For that reason, the best observation method in case of a spectrum
of sunlight is to print an image of a continuous spectrum and to mark on it the
lines seen by us, and then to compare our results with a spectrum placed below
(source: www.harmsy.freeuk.com/fraunhofer.html):
Basing on the image and
the table below try to identify the visible lines:
| Line | Element
| Wavelength [nm] |
| A -band |
O2 |
759.4 - 762.1 |
| B -band |
O2 |
686.7 - 688.4 |
| C |
H |
656.3 |
| a -band |
O2 |
627.6 - 628.7 |
| D -1, 2 |
Na |
589.6, 589.0 |
| E |
Fe |
527.0 |
| b -1, 2 |
Mg |
518.4, 517.3 |
| c |
Fe |
495.8 |
| F |
H |
486.1 |
| d |
Fe |
466.8 |
| e |
Fe |
438.4 |
| f |
H |
434.0 |
| G |
Fe i Ca |
430.8 |
| g |
Ca |
422.7 |
| h |
H |
410.2 |
| H |
Ca |
396.8 |
| K |
Ca |
393.4 |
Can you identify sodium doublet
D? These are absorption lines matching emission line of table salt, which have
been observed in part II.
The last step is to compare
your results with the ones of your classmates.
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Download instruction (pdf)
