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My two cents worth - I believe Steven has hit the nail on the head here. The lower frequency radio waves have much longer wavelengths than visible light and thus visible light will interact considerably more than radio waves through the same medium. Consequently, the velocity of visible light will be marginally slower (through a vacuum the speed is pretty much the same). I understand this interaction with the atmosphere to be associated with Rayleigh's scattering, Mie scattering and Non-selective scattering of visible light.
Cheers
Steve
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