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

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.

• possible duplicate of Why do prisms work (why is refraction frequency dependent)?
– user4552
Jul 16, 2013 at 16:01
• I notice that the OP asks about wavelength, but the answer is about frequency. This rang a bell with me. I think what the OP really means is frequency; the distinction is usually ignored. However, there is a difference. In systems that have non-local response to driving fields, the dielectric constant (or whatever) can depend on wavelength separately from the dependence on frequency. The non-local response is usually (but not always) ignorable. More in this Wikipedia article Dec 9, 2019 at 21:16

• 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. Nov 27, 2015 at 17:25