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Einstein predicted that the gravitational force can act on light. This was verified in one solar eclipse that light from a star near to the sun's disc bent due to Sun's gravity as predicted. Since Sun's corona is very hot, there should be hot gases filled beyond the corona through thousands of miles forming a convex-lens. Can it be refraction through convex lens that causes bending of light from the star rather than Sun's gravity?

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Although the corona's temperature is very high, its density is extremely small, in the order of ~10^-11 kg/m^3. Therefore coronal refraction would be negligeable.

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Will the refraction be different if Sun's corona is comprised of highly ionized gases? –  pongapundit Jun 18 '11 at 8:04
    
The corona is made up of plasma, " mainly hydrogen, but completely ionized, thence protons and electrons, and a small fraction of the other atoms in the same percentages as they are present in the photosphere." Go to en.wikipedia.org/wiki/Corona for a good overview of the subject, –  Michael Luciuk Jun 18 '11 at 13:04

Michael Luciuk's answer is right, but there's an even stronger reason for rejecting this hypothesis: refraction in the corona would be wavelength-dependent, but the gravitational bending due to the Sun has been measured over a wide range of wavelengths (at least from visible to radio) and has been found to be independent of wavelength.

Clifford Will's review of tests of general relativity is a good source for details on this. This particular test is in Section 3.4.1, but the whole thing is worth reading if you want to think seriously about this subject.

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Super Correct answer.. –  Sachin Shekhar Jul 18 '12 at 6:21

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