# Why does the refractive index depend on wavelength? [duplicate]

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Why do different wavelength get impeded more or less when in different materials? Moving with the same speed, but a longer physical distance would imply that the fields oscillate less times in the material, but I don't know why a difference in the number of oscillations would impede the wave- I don't even know why things slow down in general. Why some electromagnetic wave would slow down just because it's entering other electromagnetic fields... It would seem to me that the only factor would be time taken to physically move some electron or something in the direction of the fields... But that seems to simple of an explanation to me.

## marked as duplicate by Ben Crowell, Qmechanic♦Jul 16 '13 at 17:22

• It would mean a phase velocity faster than light yes. However phase velocities faster than light do not violate special relativity and in fact are quite common. When the refractive index is less than unity because the phase velocity is greater than $c$ this is known as anomalous dispersion. Have a look at the Wikipedia article on dispersion for more info. – John Rennie Nov 27 '15 at 17:25