Teaching Physics with the Physics Suite

Edward F. Redish

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Interstellar gas

Observations of an interstellar gas give an absorption spectrum, a part of which is shown in the upper part of the figure below. The gas is cold, such that when not excited, all electrons are in the ground state. Astronomers suspect the gas to be a type of helium, but they aren't sure. When they excite a sample of helium in a spectrometer in the laboratory, they observe an emission spectrum, part of which is shown in the lower part of the figure below.

  1. Rank the energy of photons emitted by the spectroscope gas (labeled A, B, and C) from highest to lowest.

  2. Scientist A says "line B cannot come from the same gas, otherwise we would see it in the absorption spectrum. It must be from some other substance." Scientist B says, "I think that the same substance is responsible for all 5 lines." Choose one of the answers below.
    1. I agree with scientist A.
    2. I agree with scientist B.
    3. I agree with neither of them.
    4. I agree with both of them.
    5. None of the above.
    Give a brief explanation in the space provided for your choice.

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Page last modified October 31, 2002: MP10