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Gravitational lensing question

  1. Jul 8, 2016 #1
    Hi im new to the forums. Only undertaken basic physics.
    Had a thought the other day and wanted to share it.

    Is lensing of starlight such as einstein rings ever caused by refraction?

    Could a matter density gradient theoretically cause the same effect?

    Thanks guys
    Sorry if its a repost couldn't find anything like it.
     
  2. jcsd
  3. Jul 9, 2016 #2

    malawi_glenn

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    That is how a lens works, by refraction.
     
  4. Jul 9, 2016 #3

    Ken G

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    It sounds like you are asking if there are ever any natural lenses that occur in astronomy due to matter gradients. One example is when the setting Sun looks distorted as it goes below the horizon. It isn't thought that gravitational lenses work like that, however, because there is already a natural explanation in terms of gravity effects.
     
  5. Jul 9, 2016 #4
    I understand, but is there a way of telling if the phenomena is caused by the bending of spacetime as opposed to refractional lensing due to matter gradient? How do we know it isnt from refraction?
     
  6. Jul 9, 2016 #5

    russ_watters

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    Refraction separates light by wavelength, whereas gravitational lensing does not.
     
  7. Jul 10, 2016 #6
    So are there experiments confirming no chromatic aberration? I read on another thread that the lensing from the Sun actually does have chromatic abberation.
     
  8. Jul 10, 2016 #7

    malawi_glenn

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    link/source?
     
  9. Jul 10, 2016 #8
    That's the problem. I can't find any sources for testing for chromatic abberations at all. Has it been tested for?
     
  10. Jul 10, 2016 #9

    russ_watters

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    Chromatic aberration doesn't need to be "tested for" - it is either there or it isn't. In refraction it is very noticeable:

    vt-photo-03-rw.jpg

    In gravitational lensing, it isn't there:

    A_Horseshoe_Einstein_Ring_from_Hubble.JPG
     
    Last edited: Sep 3, 2016
  11. Jul 10, 2016 #10
    Isn't dispersion different between mediums though. For example dispersing light through a prismyields much more intense dispersion than dispersing it through water or air for that matter. Perhaps the image ist high enough resolution to tell?

    Also the photo above is correcting four earths atmosphere not diffraction in venus atmosphere.
     
  12. Jul 10, 2016 #11

    russ_watters

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    Yes.
    That would be unlikely -- it's a pretty noticeable effect.
    I'm not following.
     
  13. Sep 3, 2016 #12
    Wouldn't the fact that nearly all Einstein rings are blue be evidence for diffraction?
    If not what causes this?
     
  14. Sep 3, 2016 #13
    When you write diffraction, do you mean dispersion?
     
  15. Sep 10, 2016 #14
    Yes i mean dispersion.
     
  16. Sep 10, 2016 #15
    If the image does not depend on the colour of the light there is no dispersion.
    Whatever the cause, it is not dispersion.
    What you see is the same colour as the light had before refraction.
     
  17. Sep 10, 2016 #16
    I'm not sure what you are trying to say here.

    So it is by chance that we have only ever seen blue Einstein rings in the visual range? I find that hard to reconcile, especially considering how many we have seen.

    Infact it is such a common occurance that there have been papers written attempting to answer this question.
    http://arrow.dit.ie/engscheleart2/19/

    So as of yet it is an unexplained phenomena.
     
  18. Sep 11, 2016 #17
    My physics level is various textbooks and no diploma. I don't remember such an expression as Einstein rings but I had the impression that the gravitational lensing of starlight is quantitatively calculable from some Einsteinian stuff. If the measurements line up very good with the theory then a mysterious alternative reason for the measured phenomenon would seem to be an unlikely coincidence. That would be kind of like if your plainly identifiable fingerprints at the scene of the crime had been caused by someone else. Also I don't understand your refraction theory or why you would think that starlight would be refracted or refracted in the common sense of it close to the sun?
     
  19. Sep 11, 2016 #18

    mfb

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    Please continue in this thread, having two threads about the same topic scatters the discussion and does not help.
     
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