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Gravity vs Diffraction in Troposphere of a Star

  1. May 13, 2008 #1
    How can we decide which process is causing light to bend near the surface of a star? Gravity is said to cause light to bend or deflect as it passes near the huge mass of a star. But matter in the troposphere of a star should also cause light to bend or deflect as it passes through the troposphere of the star because there should be a lot of matter (electrons, protons, H and He) in the troposphere of a star. How do I decide which is the dominant effect?
  2. jcsd
  3. May 13, 2008 #2
    I am new to relativity and you should consider some expert opinion than mine, but as this question is more of optics, let me take a chance. I will only say about the refraction point of view.

    As refraction depends upon the refractive index of the material (which depends upon wavelength), light with different wavelengths (all from same distant star) should refract to different extents. Thus, If considerable refraction is taking place at all, the distant star should be seen as images of the star in many different colors. Just like prism separates visible light. Plus, the refractive index back calculated comes out to be near 2, which is not plausible for any gaseous mixture. (There is a paper on arxive regarding this and some refraction point of view for black holes).

    Again, don't depend upon my point of view.
    Last edited: May 13, 2008
  4. May 13, 2008 #3


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    Staff: Mentor

    Correct, refraction depends on the wavelength of the light (this is called dispersion), whereas gravitational bending doesn't. Also, gravitational bending of radio waves by the sun, from pulsars or some such thing, has been studied at large angles from the sun (as seen from the earth) where the sun's atmosphere would not be involved at all.
  5. May 13, 2008 #4


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    It can be done if you have the mass of the star, and a measurement of the deflection at some known distance from the star.

    Calculate the gravitational deflection for the known mass and distance, and compare it to the actual measured deflection. If they're nearly equal, then gravity is the dominant effect. If the measurement is considerably larger than the calculation, then refraction dominates.

    p.s. While a calculation of the refractive part is possible in principle, it would require detailed knowledge of the atmosphere's density and composition as a function of altitude above the surface.
  6. May 13, 2008 #5
    Sun's atmosphere extends to earth and even beyond (Encyclopedia Britannica: "Corona"), not that it makes any difference though, as It wont achieve the necessary refractive index!
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