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Homework Help: Interference and diffraction of light

  1. Sep 7, 2004 #1
    Why is it impossible to observe an interference pattern from two adjacent incandescent globes?

    What is meant by coherant light- and how would you describe two ways of producing it?
  2. jcsd
  3. Sep 7, 2004 #2

    Coherent light can be defiened intuitively as very ordened and structured light. I mean than the wave representing the light does not really change it's properties like fasedifferences with respect to other lightwaves e.a.

    The best known example is the laser.

    An Incoherent lightsource has no order, this means that the waves propagate in different directions with all kinds of phasedifferences between the various waves. Coherent sources like the laser-resonator make sure that you get one light-wave-packet that is very ordened. All the constituant waves that make up the laser-light all vibrate in the same way, with the same constant phase-differences with respect to eachother.

    I don't get your first question though ??? What do you mean

  4. Sep 7, 2004 #3
    I think chisai_tam meant : why no interference form two different sources ? If this is the case, you answered already : two different sources cannot be coherent.

    By the way : welcome to you chisai_tam.
  5. Sep 7, 2004 #4

    True story, shuman, eeuhh, i mean humanino. Forgot you wanna be a boson-like character , meating as much superpartners as possible at once...

    marlon, two times academy-award-winner
  6. Sep 7, 2004 #5
    What's the academy award about Brando :tongue2:
  7. Sep 7, 2004 #6

    What do you mean ????

    Brando won two oscars for Over the Waterfront and The Godfather

    marlon corleone
  8. Sep 7, 2004 #7


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    "Coherent" light is light that consists of one specific wave length (in practice, a limited range of frequency). For example, burning salt produces a "yellow" flame because almost all of the light produced is in a narrow range of "yellow" frequencies.
    A "neon" light will produce light in the spectrum of neon- still fairly limited.

    In order to see interference (over more than, say, a millionth of a second!) you must have two waves of the frequency but slightly out of phase. Having light of many different wave lengths will obscure the phenomenon.
  9. Sep 7, 2004 #8
    Yes he did. I was teasing you. I like too this great actor. Did not he die recently by the way :uhh:

    pacino :biggrin:
  10. Sep 7, 2004 #9
    I am not certain this is very accurate Hallsoflvy. I think the well-defined wavelength is necessary. But still, you cannot make two different laser at the same color interfere. The coherence is
    1 in time
    2 in space

    A coherent source emits bunchs of light, with a certain time and space extension. If for instance you separate a laser light in two beams, and try to make interferences, you cannot have a too large difference between the two paths, because that would spoil time coherence.
  11. Sep 7, 2004 #10
    wikipedia will tell it better than I do.
  12. Sep 7, 2004 #11

    I also think this is wrong.

    You are mixing things up. You are referring to monochromatic-light...
    You don't need light that is completely monochromatic in order for it to be coherent.

    Indeed, Wikipedia gives a nice review of the different types of coherence.

    Last edited: Sep 7, 2004
  13. Sep 7, 2004 #12
    Yes, I also believe monochromaticity is not required. Although, the coherent sources I know are all monochromatic.
    EDIT : so I was wrong when I said "well-defined wavelength is necessary"
  14. Sep 7, 2004 #13
    Brando passed away on the first of july 2004, the day of my last exam !!!

    This is not coincidence !!!

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