Analyzing Spectra Worksheet

Part 1: Pre-exercise Exercise
  1. An emission spectrum is formed when a __________________ gas is viewed directly. An absorption spectrum is formed when a blackbody source (which produces a _________________ spectrum) is viewed through a _______________ gas.

  2. Each element produces a unique set of emission or absorption lines. An emission spectrum involves transitions of electrons from ___________ to ______________ energy states. An absorption spectrum involves transitions of electrons from _______________ to ___________ states. These transitions occur only between discrete energy levels, and thus the lines occur only at certain wavelengths and at no others.

  3. For a given atom, will the discrete wavelengths be different depending on whether we are viewing an emission spectrum or an absorption spectrum? Explain why or why not.

Consider just four of the energy levels in a certain atom, as shown in this diagram:
  1. Assume an emission spectrum and draw arrows indicating the possible transitions.

  2. How many spectral lines will result from all possible transitions among these levels? __________

  3. Which transition corresponds to the highest frequency (shortest wavelength) light emitted?
    From n = _______ to n = ________ .

  4. Which transition corresponds to the lowest frequency (longest wavelength) light emitted?
    From n = _______ to n = ________ .

In this diagram, the energy difference between states A and C is twice the energy difference between states B and C. In a transition (quantum jump) from B to C an electron emits a photon of wavelength 600 nm.

  1. What is the wavelength emitted when the electron jumps from A to B? _____________

  2. When it jumps from A to C? _____________

Part 2: Observing Spectral Lines in the Lab

  1. Your instructor will place a clear plastic box containing antifreeze on the overhead projector -- magically transformed into a large spectroscope -- and project the light from the projector, through a slit in a piece of cardboard, through the antifreeze, and through a large diffraction grating onto the screen.

    Describe the spectrum of colors before and after the antifreeze is placed in the light path. Comment on the relative intensities of each color.

  2. Your instructor will place various colored filters over the slit. Describe what happens for each filter.

  3. Practice viewing -- discuss the following observations with your teammates:
    1. Using a slide-mounted grating or a pair of "rainbow" glasses, observe any light source. Can you see the first order spectra, one on each side of the zeroth order? What do you need to do to see the two spectra? How do the colors of the two first-order spectra differ? Can you see a second order spectrum? Second orders are hard to see as they must be viewed way off to each side and are faint.

    2. Optional Use a quantitative spectroscope to observe a light bulb, a neon light, or a street light. Look about 15 degrees to the side of the zeroth image to see the wavelength scale. What wavelength range do you see?

  4. Continuous spectrum: use the colored pencils or crayons and sketch the spectrum seen of an ordinary light bulb; for example, that used in an overhead projector.

    400 nm (blue)500 nm 600 nm(red) 700 nm

  5. Make a sketch of each spectrum from the gas discharge tubes. Be sure to reflect the correct intensity of each line, the correct spacing, the relative positions, etc.

    Note: If all of the spectra look exactly the same, or you do not see a large number of red lines for neon, you may be using the spectroscope incorrectly or your spectroscope may be faulty. Check with your instructor.

    Type of GasSketch of the Spectrum
    Wavelength 400 nm 500 nm 600 nm700 nm

  6. Your instructor will insert an unknown gas emission tube into one of the power boxes. Examine the pattern and colors of the emission spectrum, mentally compare it to the gases you just observed. What is the unknown gas? ______________

Part 3: Questions

  1. Comment specifically on the similarity and differences of each of the spectra that you have observed. Include in your comments the colors you observed and how the spacing of these colors differed.

  2. Examine the following spectra:
    Artificial Solar Spectrum
    Laboratory Spectum of Iron
    What is the evidence for the claim that iron exists in a relatively cool outer layer of the Sun?

  3. How does the light that astronomers see from distant stars and galaxies tell them that the same atoms with the same properties exist throughout the universe? Why are spectral lines often referred to as "atomic fingerprints"?

  4. How can a hydrogen atom, which has only one electron, have so many spectral lines?

  5. Distinguish among emission spectra, aborption spectra, and continuous spectra.

  6. Examine the following spectra:

    What elements are present in the object that produced the "Spectrum of Unknown Composition"? Explain your method and relate this activity to the way astronomers use spectra to identify the composition of a star.