University of Michigan - Department of Astronomy

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Principles of Spectroscopy

Please read the introduction before class

Continuous Spectrum

In the photosphere (and regions deeper in the sun), the density is so high that the gas is opaque. This area produces light with a continuous spectrum.  It is radiating simply because it is hot.

For this part of the lab, you will need to get a dispersion grating from your instructor. A dispersion grating does the same thing as a prism: it splits up the light into individual wavelengths so that you can see the spectrum.

If your grating has an arrow, hold the grating so the arrow points left and right. Look through it toward a light source. You should see the "first order" spectrum off to the side. If your grating didn't have an arrow, you may also be able to see spectra above and below and at angles. These can be helpful identifying the peak color latter, but you want to focus on the spectrum to the side. There will also be fainter, "higher order" spectra farther from the light source. You want to focus on the brighter first order spectra. Watch out for spectra from other sources, such as other lights or even reflections inside the light bulb. Once you've figured out which spectra you want to observe, answer the following questions.

  1. (Prediction)What kind of light-source are you looking at: thin gas, opaque gas, solid, or liquid (circle one)?  According to Kirchoff’s laws, what type of spectrum should this produce?

  2. Observe the first order spectrum with the diffraction grating.  What kind of spectrum is it: continuous, line emission or absorption (circle one)?  How did you identify it as this type of spectrum?

Your GSI will adjust the voltage through the lamp. Watch the spectrum s/he it up, especially the relative strengths of the colors. The brightest color is the peak color and is generally the color of the filament. HOWEVER, our eyes and brains adjust quickly to the light, dimming the brightest colors and reacting poorly if the light is bright (so the filament will never look blue-green).  Because of this, you must take your first impression from the spectrum. The best thing to get the peak color is to close your eyes for a few seconds then take your first impression from the spectrum.

  1. Was the bulb hotter at a low voltage or a high voltage (circle one)?

  2. At what voltage was the bulb brightest: high or low (circle one)?

  3. List the colors you could see at:
    1. high voltage

    2. low voltage

  4. Circle the color in your lists above that appeared to be the peak at high and at low voltage.
  5. As you increase the voltage, how do the colors change (overall and the peak color)?

  6. Use your observations of the light bulb to explain which would be hotter, a red star or a blue star? You must have at least 2 pieces of evidence.

  7. Explain using both Wien's and the Stefan-Boltzmann Laws: if two stars are the same size, which is brighter, a red star or a blue star?

Line Emission

We will look at "discharge tubes," which are each filled with a low-density gas made of a single kind of atom. Running an electric current through the discharge tube gives the electrons energy and kicks them up to a high energy level. The electrons quickly fall back to their original energy level, giving off a photon with a wavelength determined by the difference in energy between the levels. Most of these photons leave the gas without interacting with other atoms allowing us to view them. This is similar to the process that occurs in the low-density, incredibly hot outer most regions of stars called the corona and in low-density, gas clouds in space called emission nebulae.

Fist look at the tube with the power off and note where there are opaque solids.  Then turn on the power and observe where the light is actually emitted.

  1. (Prediction)What kind of light-source are you looking at: thin gas, opaque gas, solid, or liquid (circle one)?  According to Kirchoff’s laws, what type of spectrum should this produce?

There should be a spectroscope set up to observe the spectrum.  A slit is aligned with the light source that allows light to travel down to the diffraction grating at the eyepiece.  The spectrum is projected onto a scale to the left of the light source. Observe the spectrum through the spectroscope. 

  1. (Observation)What kind of spectrum is it: continuous, line emission or absorption (circle one)?  How did you identify it as this type of spectrum?

  2. Observe the spectrum of one of the discharge tubes. Roughly sketch what you see, labeling the element's name and the colors of the brightest lines. Compare these to the chart of emission lines in the classroom.  PLEASE TURN OFF THE DISCHARGE TUBES WHEN NOT IN USE (but leave the sodium lamp on)


    emission chart
  3. Compare your drawing to the drawings of the different elements from the others in your class. How could astronomers use emission lines from an object?

Light Questions

  1. Describe two ways astronomers could determine the composition of a planet’s atmosphere without leaving Earth (hint, sometimes planets pass in front of stars.)

  2. Calculate λpeak for the following stars, and estimate which color dominates the visible light (therefore what color the star will appear to be.) Show at least one sample calculation below
    Star Temperature λpeak Color
    Sirius 10,000 K    
    Sun 5,800 K    
    Betelgeuse 3,000 K    

  3. Notice that the discharge tubes have different colors to our eyes. These tubes are very similar to emission nebulae. Can we use Wien's Law to tell the relative temperatures of the gas within the tubes or emission nebulae? Explain. (hint: consider what Wien's law applies to and and the temperatures associated with the colors in the above table.)

  4. The interior of the Sun is opaque, so none of the light emitted inside makes it directly to us. The region where the Sun changes from opaque to transparent is the photosphere. This is the visible surface of the Sun. Above that is the solar atmosphere, which has two layers, the chromosphere and the corona. The density and temperature of each of theses regions is given below. Use that information and Kirchoff's laws to label the diagram below with the type of spectrum that corresponds to each region, and explain how you know which type of spectrum applies to that region.
    Bottom of Photosphere: temp: 5800 K; density: 2x10-4 kg/m3
    Bottom of Chromosphere: temp: 4500 K; density: 5x10 -6 kg/m3
    Corona (average): temp: 106 K; density: 10-12 kg/m3
    Solar structure

updated: 8/30/07 by SAM

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