University of Michigan - Department of Astronomy

Version: emission only

Principles of Spectroscopy

Please read the introduction before class

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 at least three of the discharge tubes, including hydrogen. 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)

    element: Hydrogen H

    emission chart


    emission chart


    emission chart

  1. Observe the unknown element.  Sketch its spectrum, then use the chart to identify it.
  2. element:

    emission chart

  3. How could astronomers use emission lines from an object?

  4. The hydrogen atom has only one electron, but when you look through the spectroscope, you see several emission lines.  Explain how this is possible (how are several lines generated and why can you see all of them if there is only one electron per atom)

Note that you can adjust the width of the slit with a screw on the front of the spectroscope.  Observe the neon, argon or unknown discharge tubes through the grey spectroscope.  Turn the knob one or two turns to widen the slit (don't take the knob out!!), and then close it slowly while looking at the spectrum.

  1. How does the resolution of crowded emission lines change as you close the slit?

  2. How does the overall intensity of the spectrum change as you close the slit?

  3. Suppose you are observing a bright star and a faint star with your spectroscope. How would you set the slit to obtain high-resolution spectra of these stars? What would happen to the intensity? Knowing this, would it be easier to obtain a high-resolution spectrum of a bright star or a faint star? Explain.

updated: 1/16/2012 by SAM

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