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Fundamental reasoning:Electromagnetic Wave - Visible Light

  1. Apr 2, 2009 #1
    The filament of a bulb when it gets heated to some threshold level generates the visible light.

    My query:

    1. Why is it called electromagnetic then? Here, if i understand correctly, the waves are produced by the energy emitted by the transition of the electrons between various orbits of the metal filament. So where comes the magnetic field?

    2. When light rays propogate then it being an electromagnetic ray, has electric and magnetic fields pulasating perpendicular to each other. Why is it that we feel no effect of the electric field at all? Is it beacuse the field strength is very less?

    Please correct my logic if there is some flaw in reasoning.
     
  2. jcsd
  3. Apr 2, 2009 #2

    ZapperZ

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    Light produced by such filament does not come from an 'atomic transition'. It came from molecular vibration. If I have a bunch of charges and I make them oscillate up and down, I can create light. No atomic transition there, and not even the presence of atoms!

    We call it "electromagnetic radiation" because it contains electric and magnetic field"

    "We" feel no effect because "we" do not normally have a net charge or net magnetic moment that is appreciable enough. However, if you send such EM radiation to a bunch of charges, you can certainly affect it. That is who most particle accelerator works.

    Zz.
     
  4. Apr 2, 2009 #3
     
  5. Apr 2, 2009 #4

    ZapperZ

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    Now you know differently, don't you?

    Here's how you can stump the writer of that website. Since incandescent light bulbs are typically made of tungsten wire, then IF that description is correct, when one looks at the spectrum of the light made from that bulb, one will see discrete lines, and not only that, one should also see discrete lines corresponding to the tungsten element atomic energy levels!

    If you have access to a simple spectroscope (i.e. diffraction grating an a telescope set), look at the difference in spectrum between light from an incandescent bulb, and say, a discharge tube of neon. You don't have to believe what I have said. Just look at those two spectrum. Even without knowing how light from each of those sources are "made", you can immediately tell that they are very different. One has a continuous band, the other has very distinct, discrete lines.

    And if I go on to tell you that the neon discharge tube is certainly due to some atomic transition, you will then ask "So why is the light from the light bulb different?", which is exactly what you should ask howstuffworks.com.

    Zz.
     
  6. Apr 2, 2009 #5
    Zapper - That was very precisely put and has answered my query regarding the electron transition spectrum and the obvious flaw in the reasoning on that site.

    Still, can you please tell - What is it that produces light due to vibration of the molecules. I can guess its related to inter conversion of energy...but should not it lead to heating effect instaed of producing light. Again heat and light, you can say are inter linked but...What is light?
     
  7. Apr 2, 2009 #6

    ZapperZ

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    The heating is the source to cause the vibration in the solid (a tungsten is a solid). A solid like that is made of a lattice of positive ions connected via electronic bonds. One can naively picture a lattice of + and -. When the solid is heated, the lattice vibrates. Heat it more, it vibrates with larger amplitude. When charges vibrate, they produce EM radiation.

    So why is this a broad band spectrum rather than discrete? The vibration spectrum for the solid is broad and not discrete. That's why. So a series of vibrations with various continuous frequencies are possible. This relates to the continuous band spectrum of light that one gets.

    Zz.
     
  8. Apr 2, 2009 #7
    The broad spectrum from an incadescent lamp bulb is a smooth spectrum called the Planck thermal spectrum. Most of the energy is actually in the infrared. The spectrum from a fluorescent tube (and CFL) without phosphor has mercury spectral lines, primarily 436 and 546 nm.
     
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