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Cohorent light star

  1. Jul 27, 2011 #1
    for visible interference, we should have coherent source.

    if we want to take spectrum of star, need coherent light.

    i don't know that star's light coherent or not!
  2. jcsd
  3. Jul 27, 2011 #2
    If by coherent source you mean something like a laser, then no.
    To obtain the spectrum of the starlight you don't need any interference.
    You can just use dispersion through a prism. This is how the spectrometers used to work.
    Modern ones use diffraction gratings. Still don't need intrinsically coherent source, same as you don't need it in classical Young experiment or diffraction.
    Last edited by a moderator: Jul 27, 2011
  4. Jul 27, 2011 #3

    Andy Resnick

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    There are two limiting cases of the general optical coherence function: spatial and temporal. Temporal coherence refers to the spectral width of the light, while spatial coherence is a measure of the size of the source. Temporal coherence is measured using (for example) a Michaelson interferometer, while spatial coherence can be measured using a Young double-slit interferometer.

    Laser light, for example, has a very narrow spectrum (long temporal coherence), but the spatial coherence is low (speckle is visible). Starlight, by contrast, has a short coherence time (broad spectrum), but is highly spatially coherent. Just as temporal coherence can be increased by spectrally filtering the light, spatial coherence can be increased by spatially filtering the light (through a pinhole or single-mode fiber, for example).

    Measuring the spectrum of a distant star is fairly easy since the spatial coherence is much larger than the entrance slit of a spectrometer.
  5. Jul 27, 2011 #4
    Fields radiating from a star is all over the spectrum and they are prone to various effect (doppler etc) on their way as well. It doesn't make sense to me to talk about 'coherence of a star'.
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