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> >why is the speed of light for red bigger that , say, blue, when both are
> >in an optcally denser medium?
Light in a medium propagates by excitation and re-radiation.
So why different speeds for different wavelengths? The harmonic...
oscillator is being driven at a frequency below its resonant frequency.
So the motion of the oscillator is not in phase with the incoming wave,
but reaches it's maximum a little bit after the EM wave does. That means
the radiation that it puts out is also behind.
The cumulative effect of all these little delays is a propagation speed
less than c. The bluer the light, the closer it is to the resonant
frequency (which is in the ultraviolet for most glasses). The closer to
the resonance, the bigger the "phase lag," and the slower the wave travels.