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% 
% DecodingStarlight.tex
%
% Peter Newbury
% Department of Physics and Astronomy, UBC 
% newbury@phas.ubc.ca
% Twitter @polarisdotca
% http://blogs.ubc.ca/polarisdotca
%
% 12 Sept 2011
%
% This work is supported by the Carl Wieman Science Education
% Initiative cwsei.ubc.ca.
%
% Unless credit is given explicitly, all documents, graphics and images
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% Attribution-NonCommercial-ShareAlike 3.0 Unported License. 
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\begin{center}
  TA Guide for\\[1ex]
  {\huge Decoding Starlight}
\end{center}

\titlebar{Description}{}

In this activity, students use diffraction grating slides to observe the
spectra of a light bulb, the fluorescent lights on the ceiling and the
emission spectra of several gases like hydrogen, helium, neon, and
mercury (depending which tubes are available.)  The students
create their own ``catalogue'' of emission lines and then match that
catalogue to the absorption spectra of two ``mystery stars''.

\titlebar{Learning Goals}{}

After this tutorial, together with lecture materials, students should be
able to

%\setlength{\itemsep}{0pt}
\vspace{-0.5\baselineskip}
\begin{itemize}
\item distinguish between continuous, emission and absorption spectra
  by their appearance and by how they form
\item determine the chemical composition of a glowing gas by comparing
  its absorption spectrum to a catalogue of emission spectra
\end{itemize}
\vspace{-0.5\baselineskip}


\titlebar{Set-up}{20 minutes}

\medskip
Apparatus and Materials:
\begin{itemize}
\item Create 4 stations throughout the room (for example, counter at
  front left, counter at front right, counter by windows, desk nearest
  exit).  Each station needs:
  \renewcommand{\labelitemii}{$\circ$}
  \begin{itemize}
  \item 2 gas discharge tube power supplies (8 power supplies in total)
  \item 2 gas tubes (with the same gas) containing hydrogen, helium,
    neon or mercury (8 gas tubes in total) New tubes can be ordered
    through Arbor Scientific (\url{arborsci.com}) but they take about 2~weeks
    to arrive.

    \medskip
    Plug in and test the tubes. They should glow quite brightly so that when
    you look at them through the diffraction grating, you see the
    emission lines.  The hydrogen tube has a short lifetime. It should
    not be left on for longer than 30 seconds at a time.
  \item labels identifying the gases
  \item basket of crayons (4 baskets in total)
  \end{itemize}
\marginpar{\includegraphics[width=1.5in]{GasDischargeTube}}
\item each group needs a whiteboard and a couple of coloured dry erase markers
\item basket of diffraction grating slides, one for each student. These
  slides (1000 lines/mm) are available from \url{rainbowsymphony.com}.
\item incandescent light bulb you can turn on and off
\item overhead transparencies (graphics are included in DecodingStarlight.tex)
  %\renewcommand{\labelitemii}{$\circ$}
  \begin{enumerate}
  \item Page 1 (with apple sorting ``story'' and diagram)
  \item histogram of apples in the Absent Selection bin
  \item apple sorter diagram at the top, formation of
    spectra at the bottom
  \item Catalogue entry for fluorescent lights
  \end{enumerate}
\item special set of dry erase markers for the TA to use for the
  fluorescent lights demo: red, orange, green, blue, purple
\item Attach the absorption spectra of Mystery Gas~1 and Mystery Gas~2
  to the walls of the room. We create these absorption spectra by simply
  ``doing the lab'' to find the pattern of lines for each gas, then
  transfering the lines' locations with a black pencil crayon onto a
  full spectrum.  Mystery gas~1 has only one of the gases they catalogue
  to give them some success ($=$ motivation). Mystery gas~2 has 2 sets
  of lines.

  Just in case students are tempted to ``reveal the secret'' of the
  mystery gases to their friends in tutorials later in the week,
  consider creating different combinations of elements for different sections.
\end{itemize}

For the ``apple sorting'' invention activity, students will work
together in groups of 2 or 3.  After that, they'll individually create the
catalogue of emission spectra and identify the chemical compositions of
the mystery gases.


When we hand out all the pages for a tutorial at the beginning, students
often rush thru important steps because they think they have to get the
last page ASAP.  A better strategy for the students is to hand out pages
as needed:
  
\begin{itemize}
\item (1 per group) Intro and Part~1 (page 1).  You can attach this
  sheet to the whiteboard with a magnet.
\item (1 per student) Part 2 (page 2).  Hand this out to the class after
  the apple sorting activity. This page contains a continuous reference
  spectrum so this page needs to be reproduced in colour. Alternatively,
  create a sheet of reference spectra, print a number of copies (but
  only once), cut them into strips and hand out one strip per
  student. Be sure the reference spectra are the same size as the boxes
  on Page 2 (7.5~in $\times$\ 1~in).
\item (1 per student) Part 3 (page 3): Questions.  Hand this to each
  student as he/she finishes their ``catalogue'' of spectra. 
\end{itemize}
  
As usual, invite the students to form groups of 2--3 as they arrive and
ask them to come and pick up one diffraction grating slide.

\clearpage

\titlebar{Part 0: Introduction and Motivation}{5--10 minutes}

It's never a bad idea to let the students know why they are doing this,
or any other, tutorial.  Why should they care?  Why should they invest
the time and energy?  The answer should be more than the marks they'll
receive. Here's the big idea: 

\begin{quote}
  We know the colour of a gas tells us its
  temperature: blue flames are hotter, red flames are cooler. There's a
  lot more than temperature 'encoded' in the light, though. By 'cracking
  the code', we can figure out what chemicals and elements are in the
  gas.  This is what they do on TV shows like {\em CSI} and what
  astronomers do with glowing gases in space -- like nebula and stars.
\end{quote}
  
To get the students engaged with spectra (and figure out how to use the
diffraction slides), put the incandescent light bulb on the overhead and
turn on the light bulb. It might help to have the front of the room
dark.  Ask the students to look at it and get someone to describe it, to
check that they all see a continuous spectrum.  Now ask them to look up
at the fluorescent lights and get someone to describe it. It's
completely different: they'll see multiple copies of the lights, each in
a different colour. Don't overwhelm them with technical terms like
``continuous'' and ``emission''. Just let them look around.

\titlebar{Part 1: The Sunripe Apple Sorting Machine}{10 minutes}

\medskip

This activity assumes students have already seen how the electrons in
atoms have quantized orbits and that light with certain energies and
colours can be absorbed and emitted.  To remind them of this
``filtering,'' but without repeating the lecture about how light and
matter interact, we give the students an ``invention'' activity to get
them thinking about how to select certain objects from a collection.
{\bf Don't tell them} (at this stage) they're about to create a model of
atoms absorbing and emitting light.

Put up \marginpar{Page~1\\ overhead } the overhead of Page~1 containing
the apple sorting maching. Tell them
(read them) the story about how Sunripe needs special {\em Extraordinary
  Selection}$^\copyright$ apples for their pies: the
apples with diameters 60--65~mm and 80--85~mm.

\medskip
Invite the student to take 5 minutes to design the sorting machine,
using the whiteboards so they can share the results with the
class.  This won't take long and we're not expecting every group to
produce blueprints! A common invention is a sequence of holes which
allow apples below a certain size to pass through. 

\medskip
When their attention is starting to drift, get the
class' attention and ask a couple of groups (maybe groups with good but
different ideas that you spotted earlier) to share their
designs. Encourage comments from other students if they seem eager.

\medskip


Now comes the important part: linking this invention to the formation of
spectra.  Put up the \marginpar{apple sorter\\ overhead}
apple sorter overhead.  Ask them (and write on the overhead) what sizes
of apples they find in the different bins:

\vspace{-0.5\baselineskip}
\begin{description}
\item[Sunripe apples:] apples of all sizes
\item[Extraordinary Selection:] only apples with diameters 60--65~mm and
  80--85~mm
\item[Absent Selection:] apples with all sizes {\em except} 60--65~mm and
  80--85~mm.  They're likely to say, ``all the ones you don't want'' so
  prompt them to tell you the sizes: ``Right, and which are those?''
  We want someone to say ``except'' or ``everything but'' or... 
\end{description}

\vspace{-0.5\baselineskip} 

And now, the key to determining chemical
composition: looking at what's missing. Put up the histogram overhead
and give a quick description of the ``architecture'' of the graph. In
other words, don't assume that the students can interpret the graph.

\begin{quotation}
  This is a graph showing the apples in the Absent Selection bin at the
  end of the conveyor belt. The horizontal axis has ranges of sizes and
  the vertical axis shows how many apples are in each range.
\end{quotation}

Be careful not to interpret the graph for them, though. Don't say,
``...and you can see that all the 80$-$85 mm apples have been used.''
Instead, just ask them what the factory made that day.

The missing 60$-$65~mm apples mean the factory made pies that day. Pies
also use 8$0-$85~mm apples, which are also missing from the histogram
(consistency check.) What about the missing 90$-$95~mm apples? They are
used in sauce which also uses 80$-$85~mm apples (again, consistent with
the histograms.)

Now we switch to the formation and decryption of spectra. While
invention and analogy activities can be very powerful, students notoriously
fail to make the connection(s) between the analogy and the actual
phenomenon being studies. They often think they did one
thing, then another, unrelated thing. You have to explicitly make the
connections for them.

Put up the overhead showing the apple sorting and spectra
overhead \marginpar{apple sorting and spectra overhead}.  Compare the
bins and spectra in the diagrams:
\begin{enumerate}
\item The star produces a continuous spectrum of light
  with all energies.  This is like the bin of apples of all sizes
  at the factory.
\item Some of the light passes through the star's outer layers. There, atoms
  absorb light with certain energies and colours.  This gas is like the
  sorting machine (not the Extraordinary Selection bin, though -- the
  sorting machine itself.)
\item The light we observe (at the far right) has dark absorption lines
  because it contains light with all the colours/energies {\em except}
  the light absorbed in the gas.  This is like the Absent Selection bin
  that contain all apples {\em except} the ones used for apple pies.

  \medskip
  If it comes up, you can add the absorption lines are not perfectly
  black: some ``special energy'' light escapes from the gas -- either
  because it made it through or it was absorbed but re-emitted, just
  like some special apples might slip through the sorting machine.
\item If you could look just at the light coming from the gas, you would
  see an emission spectrum -- just the light with the energies (colours)
  matching the energy levels of the atoms in the gas.  That's like
  looking in the Extraordinary Selection bin.
\item The key point to wrap it up: when we see light that has passed
  through a gas, we see an absorption spectrum. By determining what
  colours are {\em missing}, we can figure out what atoms are in the
  gas. In other words, what the star is made of.
\end{enumerate}

{\em Psst! Did you notice the {\bf E}mission {\bf S}pectrum matches
  the {\bf E}xtraordinary {\bf S}election and the {\bf A}bsent {\bf
    S}election matches the {\bf A}bsorption {\bf S}pectrum? A little
bonus for the keen students...}

\clearpage

\titlebar{Part 2: A Catalogue of Spectra}{10 minutes}

\medskip

Share the main  idea for this Part with the students:

\vspace{-0.5\baselineskip}
\begin{quote}
  The key \marginpar{Hand out Page~2} to making this work is
  knowing which atoms absorb which colours.  We do that by first
  building a ``catalogue'' of emission spectra of various atoms, so when
  we look at an absorption spectrum later, we'll know which atoms are in
  the gas.
\end{quote}

It's important that the emission lines the students colour are at the
right position in the spectrum. That's why they have the reference
spectrum. However, it seems they need a little extra help, so
demonstrate using the overhead fluorescent lights.  Put up the
``Catalogue entry for fluorescent lights''
overhead \marginpar{fluorescent lights overhead} and ask the students to
look up at the lights through the diffraction grating slide.  You should
all see bright red, fainter orange, then bright green, blue and purple
lights.  Using the 5 dry erase markers, demonstrate how you look at the
red emission line, locate that colour in the reference spectrum and draw
a red line at the corresponding position in the empty rectangle. Do the
same for the other colours.  You won't get the lines in exactly the
right place but emphasize the number of lines, their approximate
location and their relative spacing.

\medskip
With that demonstration finished, invite the students to visit each of
the 4 stations around the room. They're not working in groups anymore --
each student should make the catalogue.  They should look at the gas
through their diffration grating slide and then use the crayons to
sketch the emission lines. It's true that you don't really need the
crayons and colours, black lines in the lines' locations would do, but
the students seem to be more engaged (and have a better study aid) when
they use the crayons.

\medskip

You might need to switch off the overhead lights in the room (though
you'll have to switch them back on again because we ask the students to
look at the overhead lights in Part 3...)

\titlebar{Part 3: Questions}{Remainder of tutorial}

\medskip

As students finish collecting the four emission spectra, hand
out \marginpar{Hand out Questions as needed} the
Question sheet.  When they're finished, ask them to return their
diffraction slides to the baskets.

\begin{enumerate}
\item This question asks them to determine chemical composition is an
  overly-simplified, idealized case, just so they can get the hang of
  matching collections of lines. (In earlier versions of this activity,
  the first question was the Mystery Stars question and students were
  sometimes unable to use their own emission catalogues because there
  was too much going on: colours, locations, bright coloured lines, dark
  lines,...) The answer is 2 elements, hydrogen and helium. Notice the
  distractors, though: 7 is the number of lines in the star's spectrum,
  9 is the number of unique lines in the 3 emission spectra. Yes, so of
  the lines have been shifted slightly to make them line up - again,
  part of the idealization.
\item This question is the final step in the process of spectroscopy --
  determining the composition of the gas.  If the crowd surrounding the
  mystery gases on the walls is too big, we might have to make 2 copies
  of each mystery gas.
\item This question relates the learning goal, students will be able to
  distinguish between continuous, absorption and emission
  spectra. Then we ask them to do some near-transfer of the concept: to
  identify the chemicals in the fluorescent lights, first build a bigger
  catalogue. 
\item This question is adapted from the ClassAction collection of peer
  instruction questions at \url{astro.unl.edu}. In the notation of
  continuous (C), absorption (A) and emission (E), since A+E=C, the
  correct answer is C-E=A.
\end{enumerate}


\titlebar{Clean up}{}

\medskip

We'll leave the power supplies and gas discharge tubes set up for the
duration of the week but put away the crayons and other materials.

\titlebar{Credits}{}

This work is supported by the Carl Wieman Science Education Initiative
at the University of British Columbia. See \url{cwsei.ubc.ca} for more information.

Unless credit is given explicitly, all documents, graphics and images
are licensed under a Creative Commons BY-NC-SA
Attribution Non-Commercial Share-Alike 3.0 Unported License. 

If you have comments or suggestions, please contact

\begin{quote}
  Peter Newbury\\ 
  Department of Physics and Astronomy, UBC\\
  email: \url{newbury@phas.ubc.ca}\\
  Twitter: \url{@polarisdotca}\\
  web: \url{blogs.ubc.ca/polarisdotca}\\ \\
  14 September 2011\\
\end{quote}

\end{document}

