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Hi,
I would say the speed of an electromagnetic wave NOT in a vacume does depend on the wavelength AND the refractive index. If this was not the case then there would be no rainbows as the different wavelengths would all have the same speed and so would not diverge.
Radio waves generally are thought of as electromagtnetic waves of enormous wavelengths (compared to light) they dont refract as much in a transparent medium such as air, water, glass, diamond and so would be faster than light in any given medium; EXCEPT in a vacume.
Cheers
Paul
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