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Some questions about spectroscopy

  1. Apr 15, 2013 #1
    When we want to view the spectrum of light emitted by a certain gas (via spectroscopy) how much voltage do those gas discharge tubes end up having? I know they are very high voltage, but how high exactly?

    Now if I were to look at an incandescent light bulb through a spectroscope, I would see the full, continuous spectrum of light. Since light bulbs do contain some inert gas (like krypton), why don't we see the spectrum emitted by the krypton itself? Is it because light bulbs are usually low pressure? Is it because the voltage is only 120 V?

    This is my guess: I know that an LED will emit a much "fuller" and "clearer" spectrum than a regular light bulb will. I am guessing that since the gas inside a light bulb is low pressure, there isn't enough of it to emit that specific gas' spectrum only. I would also guess that because there is insufficient voltage to cause the electrons to jump around in the energy levels of the krypton, we also don't see the discrete lines. Since an LED does not use a gas filled tube like a bulb does, the spectrum is much more complete. The low pressure gas inside a light, is not enough to show its discrete emission spectra. Moreover, the fact that we only have a small amount of that gas, this is what cases the light from a bulb to be less complete than the light from an LED.

    If someone could help me with my questions I would appreciate it.
  2. jcsd
  3. Apr 15, 2013 #2


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    Staff: Mentor

    The electricity doesn't go through the gas, so there is no electrical excitation of the gas.

    The gas has nothing to do with it. You would get the same result is the filament was surrounded by a vacuum. The spectrum depends on the emission proporties of whatever is actually making the light.
  4. Apr 15, 2013 #3


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    Staff: Mentor

    The Franck-Hertz experiment shows that a few Volts are sufficient, but if you want to light a room, devices with several hundred Volt are more practical.

    Krypton is not bombarded by electrons there, light is generated by the heat of the wire. It might be possible to see some absorption of Krypton, as light has to pass through it. I don't know if there are lines in that range.

    What do you mean with "fuller" and "clearer"? LEDs emit light in some specific frequency range only.
  5. Apr 15, 2013 #4
    I see now. The gas is not electrically excited in a light bulb. As for seeing some absorption of krypton, the concept makes sense (that's how one would view the absorption spectra for gases, send white light through a gas and then the light that is not absorbed by the gas is diffracted through a diffraction grating).

    Someone told me that in the aforementioned case, the gas should be cold and not electrically excited. Is this true?

    Some of the images I have seen of the spectrum emitted from an LED vs an incan. bulb have been different. Of course both give off the full spectrum but sometimes one looks more continuous in terms of the colour gradient/transition.
  6. Apr 15, 2013 #5
    One single LED does not give off the full spectrum. If by this you mean full visible spectrum.
    Maybe you mean an LED lamp? (these bright "white" ones)
    Even then, they seem to have a peak in the blue region.
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