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Difference between coherent light and polarized light

  1. Jan 30, 2011 #1
    What is the difference between coherent light and polarized light?
  2. jcsd
  3. Jan 30, 2011 #2
    I found the answer in this 2 links:
    http://www.matter.org.uk/schools/content/interference/coherent.html [Broken]

    If I understand this correctly coherent light means that the waves are syncronized(in phase) and they have exactly the same wave length.

    Polarized light means the waves have only one direction - the direction of the polarization.
    In polarized light the waves do not have the same length and the waves are not in phase.

    Coherent light is polarized, but polarized light is not coherent.

    Is this correct?
    Last edited by a moderator: May 5, 2017
  4. Jan 30, 2011 #3
    Pretty much, although I'm not sure you can get exactly the same wavelength. I think there will always be some spread, even in lasers, so you'll get decoherence after some length: this length is not infinity. According to Wikipedia:

    " The most monochromatic sources are usually lasers; such high monochromaticity implies long coherence lengths (up to hundreds of meters). For example, a stabilized helium-neon laser can produce light with coherence lengths in excess of 5 m. "

    So if I'm understanding this right, Star Trek lasers can't go very far to shoot ships (after some distance though, I guess the laser would reappear after all the destructive interference - recoherence?)

    Besides being mostly of one wavelength, the waves being generated by the source also have to have a constant phase relation, as you say. It can't be unsteady like a lightbulb.

    Also, there is the concept of spatial coherence (which to me seems very fundamentally different from the condition of being monochromatic), that if I understand it correctly, basically says your wave can't vary too much in phase if you walk along a wave envelop. This is most important for a double slit experiment (you want the light at the two slits to be mostly in phase), so my guess is that you ensure this by placing the light bulb far away from the double slit (e.g., distance stars product spatially coherent light - if something is far away it is practically a point source).

    This is mostly correct, but you can have circular or elliptical polarizations: the tip of the electric field need not be confined to a line.

    This is not true. They can be of different length and need not be in phase.

    This is not true either. Coherent light can be unpolarized, and polarized light can be coherent.
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