Labs from YSI 94 :
Stellar Spectroscopy.

Luisa Rebull and Dr. Al Harper
CARA Yerkes Summer Institute, August 1994

This is the staff copy of the lab and other brainstorming/reflections.

Equipment to be gathered and prepared ahead of time:

Colored cellophane (we had only green & purple after an exhausting search of Chicago); hand-held spectroscopes, tubes of gas, and power supplies from the physci labs. Desk light or other directional plain- light light bulb source. Data sheets (attached). Also get laser (for further discussion), a copy of an MK spectral atlas, and the gadget that fits onto the eyepiece of the 24" that allows you to easily see spectra of bright stars.

Need a room big enough for easy movement, suitable for having kids and high voltage power supplies together in the dark, with plenty of three-prong plugs.

Experiment, part 1

Ask them what colors are in the rainbow and if they can recite them in order. (This week they couldn't, but when we were doing the resistor codes last winter, they knew it well...?) They know what a prism does - tell them that we're going to give them a tool that does the same sort of thing, breaks up the light into components. Distribute spectroscopes and work with pronunciation of the word spectroscopes. Have them look first at the plain light. Make sure everyone can see the rainbow, is looking through the correct end, etc. Try taking tube off and using just the grating if they have trouble. Light pollution is a problem - used screens to block adjacent tubes.

Experiment, part 2

Tell them to pick a gas tube and look at it. Do the colors still go smoothly from one to another? They don't - distribute data sheets and explain what to fill in where. They had a hard time with this. Added (to this edition) explicit place for "orange", etc; they had trouble when we just had "red" and "violet." (...difficulties with abstract thinking that have been observed before....) Not sure how to improve data sheet, except maybe to have instead a series of questions like "At the pink tube, how many green lines do you see?" etc.

Show how diffraction grating works with monochromatic light (laser). Show how to rotate grating if they have problems finding rainbow.

Point out that undiffracted light has color. (Are there any purple lines in the neon?)

Experiment, part 3

Now distribute pieces of cellophane. Work with the gratings and white light from a regular tungsten light bulb. What does the green cellophane do to the light? The purple one? Some people have a REALLY hard time with this. ("It just makes it dimmer.") They don't understand what the filters should do. Try folding over cellophane several times. I'm not sure what the problem was here. Light pollution? Beats me. This one takes a little while to do because of these problems.

Some of the kids are smart enough to really work hard at figuring out why the two lights look different. I don't think it's possible for them to guess the real answer: tungsten solid versus gaseous elements. (too little chemistry/quantum physics.) Several of them have had chemistry, though. These same kids get the idea that fluorescent lights look different than normal light bulbs even without the spectrograph. They also recall Na or Hg streetlights.

Experiment, part 4

Point out that each element has a different signature like these here. Distribute MK atlas sheets and ask them what elemental symbols mean (many of them get it). (Gloss over emission/absorption -- unnecessarily confusing.) Point out that this is what real astronomers do. Go upstairs to view the real spectrograph. Talk about other kinds of light and lines in those wavelengths.

Experiment, part 5

If it's clear, go up to 24". Point out Antares, Vega on the way. They don't have too much trouble seeing that the stars have color, just like the tubes. It was never clear enough to do the actual observing, however.

Tried one night to get the spectra of fireflies... squish 'em - they glow brightly for a long time that way.

Things they should take away:

  1. White light is made up of colors.
  2. Diffraction gratings, like prisms, work to reveal the colors.
  3. Each element has a "finger print" of emission lines.
  4. Because each element has this signature, a collection of elements lumped together can be deciphered by a patient person who has looked at the signature of each element separately before - or has access to this data that someone else took.
  5. Light travelling to us through space to us betrays the kind of material it came from (and through).
  6. This is not just some pre-canned lab - this is what real, working astronomers do.

Further things:

  1. This is how you can look for stars similar to the Sun, by comparing spectra.
  2. How does infrared and ultraviolet light fit into this? Might there be lines in those wavelengths too?
  3. How do fluorescent lights differ from regular lights?
  4. How does emission and absorption work? (Maybe too heavy for these kids?)

Sample data sheet

Spectroscopy Lab    			Name:_______________________
(Harper/Rebull)

Pick a tube.  Draw what you see.

-------------------------------------------------------------------------
|                                                                       | 
|                                                                       | 
|                                                                       | 
-------------------------------------------------------------------------

  Red     Orange     Yellow      Green      Blue      (Indigo)     Violet


Pick another tube.  Draw what you see.

-------------------------------------------------------------------------
|                                                                       | 
|                                                                       | 
|                                                                       | 
-------------------------------------------------------------------------

  Red     Orange     Yellow      Green      Blue      (Indigo)     Violet



Pick another tube.  Draw what you see.

-------------------------------------------------------------------------
|                                                                       | 
|                                                                       | 
|                                                                       | 
-------------------------------------------------------------------------

  Red     Orange     Yellow      Green      Blue      (Indigo)     Violet


Pick another tube.  Draw what you see.

-------------------------------------------------------------------------
|                                                                       | 
|                                                                       | 
|                                                                       | 
-------------------------------------------------------------------------

  Red     Orange     Yellow      Green      Blue      (Indigo)     Violet


Important Disclaimers and Caveats: