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Thread: Spectroscopy: Why do we see spectral LINES, not something like circles or triangles?

  1. #1 Spectroscopy: Why do we see spectral LINES, not something like circles or triangles? 
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    Just curious. We were looking at the spectral lines of sodium and I was just wondering why the spectra occur as lines, not any other shape.
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    Quote Originally Posted by girlinphysics View Post
    Just curious. We were looking at the spectral lines of sodium and I was just wondering why the spectra occur as lines, not any other shape.
    It's merely the way the spectrograph is formed. Light enters a prism through an entire face of one side of the prism. The trajectories of all the photons entering the prism face are parallel. Light of a particular wave length gets bent by the prism by a certain amount which is determined by the prism. That means that the light of the same frequency forms a line.
    Last edited by Physicist; 09-24-2014 at 03:29 AM.
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    Quote Originally Posted by girlinphysics View Post
    Just curious. We were looking at the spectral lines of sodium and I was just wondering why the spectra occur as lines, not any other shape.
    They are not lines, they are bands, see here.


    Sodium emits multiple frequencies (wavelengths). The speed of light in a medium is function of its frequency (not true in vacuum). Since the light emitted by the sodium has to travel through the glass prisms of the spectrograph, different frequencies travel at different speeds and they get separated, forming the vertical bands. All close frequencies "band" together.
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    Quote Originally Posted by girlinphysics View Post
    Just curious. We were looking at the spectral lines of sodium and I was just wondering why the spectra occur as lines, not any other shape.
    I am interpreting your question as asking why the spectral lines are spectral lines, and not why a spectroscope properly reflects the fact that they are lines.

    The answer is that these spectral lines correspond to transitions between electron energy levels. Electrons in an atom are allowed to have only specific energy values, and each atom has its own characteristic set of such energy levels. As electrons are induced to make a transition from a high-energy state to a lower one, the energy difference is given off as photons. Since the energy differences are a discrete set, you get lines instead of a continuum.

    These spectral lines allow scientists to determine with great precision what atoms are in a sample of unknown composition. And the same physics allowed us to discover helium -- in the light from the sun, where previously-unknown lines showed up. Later, we found helium here on earth, creating less dangerous birthday balloons. But its discovery in sunlight is marked in the name, derived from Helios, the sun god.
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