Gas Tube Spectroscopy - 1 of 6
Gas
Spectroscopy
Tube
Brief Summary
Very often, a substance can be energized and made to glow. Though solids at the same temperature
glow with the same color (lava, red hot pokers, etc.), low pressure gases glow with their own
distinctive colors. To the naked eye, it may be hard to tell one color from another. But passing the
colored light through a diffraction grating breaks it up into its component colors - or spectrum. This is
an activity where visitors can view glowing gases through diffraction grating glasses and see the gases'
spectral “fingerprints.” This same technique is used to identify the chemical components of stars.
Equipment Required
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Spectrum Glasses
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8 Spectrum Photos
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Power source
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Spectrum Charts
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Protective shield
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Bohr Atom Charts
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Gas Tubes
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Colored Filters
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Spectrum Tube Cart
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Gloves or other to remove
tubes when hot
Main Teaching Points
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The rainbow of colors that we call a solar spectrum is really a collection of about 300 individual
and distinct lines of color merged to give us a continuous smear of color. These individual lines can
be separated out of the spectrum with certain tools such as gratings and prisms.
When energized, every kind of gas glows with a unique pattern of colored lines (called its
"spectrum") which can be used to identify it. A similar technique is used to identify the
composition of elements in stars.
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Our eyes blend the various spectral lines together to give us one color impression such as “cherry
red.” When analyzed through a diffraction grating, two identical-looking color impressions may
turn out to have very different spectra.
Educational Strategy
This is an on-going exploration in which you coach visitors through a series of discoveries, not a
demonstration. Under the guidance of a Museum Galaxy Guide, visitors can follow their own
curiosity and try a variety of different experiments. The attractive nature of the glowing gases appeals
to a variety of people so this activity can be appreciated by different people in their own way
(scientifically, aesthetically) and at their own level.
Set Up
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Position Gas Tube Spectroscopy cart and plug in power cord.
Place tube(s) in power source and push ON button.
Hand out spectrum glasses to visitors.
Use gloves, paper towel, or other barrier to remove gas tubes since they will be hot if left on for
more that a few moments.
Turn off tubes when not in use since they have very short life expectancies.
Suggested ways of presenting activity
Try this:
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Place a variety of gas tubes in the power source.
Let visitors explore the different spectra using the spectrum glasses.
Have visitors describe the spectra.
How do the spectra of various gases differ?
Or try this:
 Have visitors try to identify which gas is in the glowing tube by matching it with the
spectrum photo cards.
Or this:
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Energize two spectrum tubes side by side at the same time. Point out how difficult it would
be to decipher which TWO substances are glowing.
Now show a spectrum chart of an actual star to demonstrate its complexity.
Operating Tips
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Tubes get hot. Museum Galaxy Guide must handle spectrum tubes and equipment, not visitors.
Safety Feature: The clear shield must be in place or else power will not turn on.
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Questions and Answers
What causes the gases to glow?
All substances are made up of ATOMS (or ions or molecules which are, in turn, derived from atoms).
The atoms themselves have ELECTRONS in them. When
you add energy to an atom (by heating it or putting
electricity through it, for example), an electron can jump to a
higher ENERGY LEVEL. This is the way that an atom can
temporarily store energy. Amazingly, though, an atom
cannot absorb just any arbitrary amount of energy, but only
certain exact amounts. (It’s like a candy machine that only
takes quarters. You can’t put in 17¢.) After a short amount
of time, the electron loses energy and falls back to the lower
energy level. This surplus energy is expelled by the atom in
the form of a PHOTON of light. Since the color of the
photon corresponds to the amount of energy it carries, you
can tell how far the electron has fallen (from one energy
level to another) by the color of light it gives off. For
example, blue photons carry more energy than red ones. (In
actuality, electrons don’t orbit like little planets, and they
don't "jump" and "fall." This is just a mental picture.)
Demo: show visitors the Absorption and Emission chart.
Why are there lots of colored lines in a substance's spectrum?
Each substance only allows its electrons to be at very
specific energies. (This simplified diagram shows the four
"allowed" energies for an imaginary atom.) Suppose that
the electron started at the lowest energy level or
"GROUND STATE" (inner-most circle) and was
energized with the right amount of energy to jump to one
of the higher states. When it falls back to a lower state, it
gives off a photon. As you can see, there are a number of
possible ways this might happen. Each possible "drop,” or
TRANSITION, corresponds to a differently colored
photon, depending upon how much energy was lost in the
drop. A small drop in energy produces a lower energy
photon (red), while a large drop produces a higher energy
photon (purple).
When you energize an actual gas, there are an enormous
number of individual atoms or molecules that you are
energizing. For some reason, certain transitions are more
likely to occur than others. The more often a particular
transition occurs, the more photons of that color are given
off, and the brighter that colored line is in the spectrum.
In summary: the color (and position) of the lines in the substance's spectrum corresponds to all of the
possible ways that its electrons can drop. The brightness of the lines corresponds to how often that
particular transition occurs.
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Demo: show visitors the Spectral Lines chart. Ask which spectral lines correspond to which
transitions. The graph on the bottom of the chart is a more accurate way of presenting the spectrum.
The height of the peaks represents the brightness of the colored lines.
Why do different gases glow with different colors?
Each atom or molecule has its own arrangement of protons, neutrons, and electrons. As a result, each
has a different set of allowable ENERGY LEVELS that electrons might occupy, and so, a different set
of colors in its spectrum. Your eye and brain combine all of the colors in a substance's spectrum into a
single color which you perceive.
Demo: Suggest that visitors experiment with the Spectroscopy interactive where they are able to mix
colors to see how their eyes/brain perceives various mixtures.
Why do some spectra have only a few lines and others have whole bands of color?
In the illustrations above, we have shown only one electron and its various possible energy levels.
This resulted in a spectrum with only a few lines. Now imagine an atom with many electrons. There
will be many more possible energy levels, and many more electrons jumping from energy level to
energy level. This will cause many more spectral lines.
Taking this a step further, when atoms combine to form molecules possible energy levels arise not only
from each of the atoms, but also from the interactions between the atoms. The result is often that there
are so many lines in the spectrum that they almost seem to blend into a continuous band. Compound
this with the fact that a molecule can vibrate and rotate which can also produce photons, and you have
a spectrum that can be quite complicated.
As a rule of thumb, simple atoms (like Hydrogen or Helium) have simpler spectra than elements
higher on the PERIODIC TABLE, and molecules have more even more complicated spectra.
Demo: Show visitors glowing gas tubes and have them guess which are produced by glowing
molecules and which by glowing atoms.
Isn't it true that everything you've said so far applies to non-visible light as well as visible light?
I mean, doesn't a glowing atom or molecule give off light in the infra-red region, or the ultraviolet region? And isn't it true that scientists study the specta of astronomical objects in many
different wavelengths, like x-rays, or microwaves, not just the visible-light wavelengths?
Yes.
Demo: Suggest that visitors check out the Thermal Imaging interactive to experiment with infra-red
light. They should also check out the Sun in Many Wavelengths interactive to see other types of light
that the Sun gives off in addition to the visible light we are familiar with.
Other Cool Stuff to Try
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Use the spectrum glasses to look at other light sources around the room.
With your spectrum glasses on, look at glowing gas tubes through colored filters. Notice how the
filters don't add any color; they actually subtract parts of the spectrum! For example, a green filter
subtracts all but the green light. (The graphs that go with each filter show which colors will pass
through and which will be blocked.) Gases that surround stars can also subtract parts of the star's
spectrum. This results in black lines, called FRAUNHOFER LINES.
Try using other spectroscopes to look at the gas tubes.
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Fast Facts
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In 1666, Isaac Newton was the first to discover that sunlight could be broken into a rainbow of
colors, then recombined into its original white color. Because of the magical way the rainbow
appeared on his wall, he coined the term "spectrum" which is the Latin word meaning apparition
or ghost.
Joseph von Fraunhofer discovered the dark lines in the Sun's spectrum in 1814 and realized they
were gaps of missing colors. He also invented the diffraction grating which is a much more
efficient than using a prism to create spectra.
The science of spectroscopy was created in 1859 by Gustav Kirchoff and Robert Bunsen. They
invented a device that used a prism to break up the light of substances being burned in a Bunsen
Burner.
Potential Problems
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Gas tubes can get very hot. Be careful and use gloves.
Gas tubes can break. Be careful during handling of tubes and transportation of cart.
Background materials (websites, videos, articles, digital collections links)
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http://antwrp.gsfc.nasa.gov/apod/ap000815.html The Solar Spectrum
http://lasp.colorado.edu/home/education/k-12/project-spectra/ CU LASP interactive site using real
data from Cassini probe
http://web.mit.edu/spectroscopy/overview/index.html MIT George R. Harrison Spectroscopy
Laboratory website
http://mo-www.harvard.edu/Java/MiniSpectroscopy.html Interactive site -- Java required
http://www.spectroscopyonline.com/spectroscopy/article/articleDetail.jsp?id=381944&sk=&date=
&pageID=1 Spectroscopyonline.com
http://www.amateurspectroscopy.com/
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Self assessment suggestions
After demonstrating the GAS TUBE SPECTROSCOPY several times, complete the checklist, then
highlight the box in the rubric that best describes your performance. Have your team leader observe
your demo then complete an identical rubric. Discuss your presentation technique with your team
leader along the lines of the rubric.
Assessment for GAS TUBE SPECTROSCOPY
DATE________ PRESENTER_______________
A. Checklist of pre-requisite skills
1. Can set up & put away gas tube spectroscopy cart & its various props and print materials.
2. Knows (and always abides by!) safety rules for the high voltage source and the hot, fragile
gas tubes.
3. Fully understands the connection between 1) the glowing gas in the tube 2) the bright colored
lines that make up the spectrum and 3) the diagram of the atom representing the electron
transitions.
4. Can interpret & explain the graph showing how a color filter subtracts light from a spectrum.
5. Can use the gas tubes, glasses, charts and filters fluently during an explanation.
B. Rubric for GAS TUBE SPECTROSCOPY ACTIVITY
QUALITY
LEVELS 
OK
TRAITS 
Knowledge of the science
Can answer visitor questions
correctly
Effectiveness using props
Can make a smooth presentation
using each prop at some time
during the demo
Presents a step-by-step explanation,
allowing visitor to digest one
concept before going onto another
one.
Explains how each device, chart or
diagram on the cart relates to the
other parts
Educational strategy
Linkage of conceptual
elements
Comfort level with open
ended questioning
Guides the visitor to try standard
variations of the demo
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EXCELLENT
Can go beyond visitor question
and add interesting facts gleaned
from various resources
Can do the demo using several
different approaches and can
incorporate props in any order
Uses a step-by-step approach and
actively insures visitor is ready to
move on by asking appropriate
questions
Tests for misunderstanding or
poorly made connections in
visitors’ mind by asking relevant
questions
Wonders out loud and allows
visitor to figure out the answer by
trying variations of the demo