Spectroscopy in Astronomy: Emission Spectra
Equipment: (Shared among all lab groups)
Blue spectrometer (one for each lab group)
Spectrum tube power supplies (5000 volts—be careful!)
These will be set up around the room, one with a hydrogen gas tube, and three
others with unknowns.
Sodium light (up front)
Red reading lamps, set up next to the discharge tubes.
Your answers to questions, descriptions of your procedures, data (including tables),
and results will need to be written up and handed in on a separate lab report. Make
sure to include an overall discussion and conclusions for the lab.
Introduction:
A spectroscope is a device used by astronomers (and others) to separate light into its
various color components. Basically, there are two types of spectroscopes: one uses a
prism usually made of glass while the other uses a diffraction grating which is made of a
plate of glass with very fine and accurately placed parallel scratches on one face. The
grating or prism in a spectroscope is called the dispersing element.
The purposes of this experiment are:
(i)
To familiarize yourself with the use of a very simple spectrograph,
(ii)
To determine how accurately you can measure wavelengths in an emission-line
spectrum, and
(iii) To identify some unknown gases from their spectra.
Procedure:
Divide yourselves into groups of 3 or 4. Over the course of the lab, make sure that
everybody has a chance to use the spectrograph, and take some data. Begin by
examining your spectrograph. It should look roughly like the following image, when
viewed from above:
scale
slit
eyepiece
At the eyepiece end, what looks like a clear piece of plastic is actually a transmission
diffraction grating, with thousands of grooves cut into it. Much of the light that hits the
grating passes straight through, and you are able to see through it. Some of the light,
however, hits the grooves and scatters in all directions. In certain angles, light coming
from different grooves interferes constructively. This happens at different angles for
different wavelengths, with the result that the grating spreads the light out into a
spectrum. The process is sketched below, for two grooves (the gratings on the
spectrographs actually have thousands of grooves).
Bright points
appear here
where the
difference in the
paths between the
observer and the
two slits is an
integer number of
wavelengths.
Planar light waves
coming in.
Screen with
two slits
Light scatters
off of slits
The calibration of the spectrographs is not perfect. Before proceeding, you’ll need to see
how well calibrated yours is. First, look through the eyepiece of the spectrograph, while
aiming at the room lights (everyone in your group should do this). You will see two
scales. One gives photon energies in electron volts (eV), and the other gives photon
wavelengths.
1) What are the numbers on the wavelength scale?
Two units are commonly used when measuring the wavelengths of visible light. Most
physicists in the world use nanometers (nm), where 1 nm = 10-9 m. The unit most
commonly used by astronomers is the Ångstrom, where 1 Å = 10-10 m.
2) Based upon the numbers that you see on the scale, what units do you think that we will
be using to measuring wavelengths in this lab?
3) What are the smallest divisions on the scale? How precisely do you believe that you
will be able to make wavelength measurements using this scale (give your expected
precision as a number)?
Now, make sure that the slit is aimed at the overhead lights. Do you see a spectrum
projected against the scale on the side? If not, then rotate the eyepiece that holds the
diffraction grating until the spectrum of the room lights is projected on the scale. Make
sure that everybody in your group is able to see the spectrum of the lights, and that you
individually record the answer to the following question:
4) Looking at the overhead lights, you should see a continuous spectrum, with some
brighter lines at certain wavelengths. Record the wavelengths of the brightest lines that
you can see. Compare them to the location of the brightest lines of Mercury given below.
How closely do they compare. Quantify your answer. That means to use numbers:
“pretty close” doesn’t qualify as a comparison!
Mercury Gas
Color
Wavelength
(nm)
Violet
405
Violet
436
Blue
492
Green
546
Yellow
577
Yellow
579
Orange
607
Red
691
Now make another check of the location of the scale, and learn to use the spectrograph in
the dark. Once everybody has completed step 1, the room lights will be turned off, and
the instructor will turn on the sodium light.
Sodium emits a doublet of lines (two lines spaced very closely together), at 589.0 and
589.6 nm. These will appear as a single line in your spectrograph. (The pureness of the
emission explains why things look so odd when illuminated by streetlights that use low
pressure sodium gas.)
5) At what wavelength do you see the sodium doublet? What is the percentage difference
between what you see and the actual average doublet wavelength of 589.3 nm?
6) Tabulate the difference between the wavelengths recorded on your spectrograph and
the actual wavelengths. Fill in the following table for all the lines that you could see
(transcribe it onto your final lab report). All units should be in nm. For the difference,
subtract the true wavelength from your measurement (so, if your spectrograph reads
wavelengths as being too small, the difference will be negative). For the ratio, divide the
true wavelength by your measurement (so, if your spectrograph reads too large, the ratio
will be less than one). For the final row, average the results of your differences and
ratios.
Element
Hg
Hg
Hg
Hg
Hg
Hg
Hg
Na
Average
Actual
Wavelength
(nm)
Measured
wavelength
(nm)
-------
-------
Difference
Ratio
7) Go to the lab station with the power supply holding the hydrogen gas tube, and turn it
on. In order to see the scale better, it will help to project the red reading lamp either at
the wall or onto a sheet of paper such that the red light illuminates the scale of the
spectrograph. Don’t shine the red lamp directly at the spectrograph—it’ll be too bright,
and will swamp your spectrum. You will probably see 3 lines in your spectrograph.
Complete the following table for the lines that you can see:
True wavelength
(nm)
Color
656.3
486.1
434.1
410.2
Red
Aqua
Violet
Violet
Measured
wavelength
(nm)
Correction 1 Correction 2
Which is
better?
For “Correction 1” take your measured wavelengths and subtract the average difference
that you found in step 6. For “Correction 2” take your measured wavelengths and
multiply by the average ratio that you found in step 6. In the final column, decide which
of the two corrections gave you a wavelength that was closer to the true wavelength.
Hopefully (but not certainly!) it will be the same for all of the lines. Don’t worry if it
isn’t. Pick as your best the one that works better for more lines.
8) Now try to identify some unknown elements using their emission spectra. There are 3
or 4 unknown tubes at stations around the lab, labeled by number. Look at each of the
unknowns with your spectroscope, write down how many lines you can see for each, and
your best measurement of the wavelength of each one. Keep a record of these on a
separate sheet of paper, and hand it in as part of the data section of your lab report.
9) As you work on step 8, make your best guess as to the identity of each of the unknown
elements, and record them, along with the reasons for your choices. To do this, compare
your results with the lists below, with charts in your text, the color charts that your
instructor will have, the charts on the web page listed below, and with the chart in the lab
room. Visual comparisons will be the easiest, and so the list of wavelengths is
recommended only as a last resort. Hint: the unknowns will be from among helium (He),
oxygen (O), nitrogen (N), neon (Ne), argon (Ar), xenon (Xe), and mercury (Hg).
In making your comparisons, keep the following points in mind:
a) You won’t see all of the lines that are either shown on the images or listed
below—our spectrographs are too crude for that.
b) If you see lines in your spectrograph that are either not listed or are not shown,
then you have a bad match, and should look at another element.
c) Don’t worry too much about the extreme red and violet wavelengths. Our eyes
aren’t very sensitive to wavelengths near 400 nm and 700 nm, and so you may not
see something there that appears on the list or on the images. This problem varies
from person to person, but gets worse with age—see for example the images at
the top of the web page, where the instructor is comparing his perception of the
spectra with that of his students.
d) If you are comparing the numbers that you measure with those on the lists below,
keep in mind the differences and corrections that you found in steps 6 and 7
above. If you’d like some extra credit, tabulate your measured wavelengths, and
include the best correction that you found in step 7.
Wavelengths of some of the elements. All wavelengths listed below are in Ångstroms.
Divide by 10 to get the wavelengths in nm. You will probably find it easier to use a
visual representation, which can be found at http://astro.u-strasbg.fr/~koppen/discharge/.
Your instructor will have a color version of this as well. Also look at the chart in the
classroom, and at the chart that your instructor will have, which is based on the figures
from the above web site.
Helium (He)
4388 Å 4471 Å 4713 Å 4922 Å 5016 Å 5048 Å
Argon (Ar)
4610 4658
6677 6753
5876Å 6678 Å
4727
6871
4736
6965
4765
4806
4880
4965
5142
6115
6172
Krypton (Kr)
4274 4320 4355
4847 4946 5022
5871 5992 6420
4470
5087
6456
4577
5126
6570
4619
5208
6904
4659
5309
4739
5333
4766
5468
4825
5570
4832
5682
6143
6929
Neon (Ne)
4569 5852
6164 6182
7032
5873
6217
5945
6267
5965
6334
5975
6383
5976
6402
6030
6507
6074
6599
Oxygen (O)
4593 4638
5436 5577
6653
4645
5958
4662
6046
4676
6106
4700
6157
4925
6261
4943
6370
5330
6454
Xenon (Xe)
4501 4525
4917 4923
5895 5931
6473
4583
5028
5934
4624
5393
6164
4671
5460
6178
4697
5696
6180
4734
5697
6182
4793
5716
6198
4807
5824
6201
4830
5825
6318
4843
5875
6470
Nitrogen (N)
3943 3998 4060
4574 4649 4667
4095
4724
4142
4815
4201
4917
4270
4976
4344
4355
4417
4490
Since nitrogen is a molecule, its spectrum consists of bands rather than lines. This is due
to rotation of the molecules. In the visible part of the spectrum, the most prominent
structure is the First Positive series, with about 30 regularly spaced bands in the region
5000-7000 Å. Only the band heads of the less intense Second Positive series are listed
above.
Prelab Questions:
1) Describe what is meant by constructive and destructive interference of light waves.
2) If two rays of light have a path length difference of 500 nm, what is their wavelength
if they constructively interfere (show your work)?
3) What happens during the creation of the emission spectra which you observe from the
discharge tubes?
4) (show your work, or describe how you arrive at your answers)
a) If an electron is in the ground state of a hydrogen atom, what wavelength photon
must be absorbed in order to promote the electron up to the third energy level?
b) If an electron jumps from the third energy level down to the second, what
wavelength photon is produced? What color will this photon appear as?