Why does the amount of Refraction depend on wavelength?

[Fiona Rozario]

I understand that electrons of a material have a natural frequency of vibration and the refractive index results from the phase difference between the incident light's field oscillations and the field oscillations of these electrons. (https://physics.stackexchange.com/questions/71126/why-does-the-refractive-index-depend-on-wavelength) So the refractive index will be the least for a light frequency which is close to the natural frequency of the electrons of that material.

So how is it that refractive index of materials is the least for red and most for violet? Does this mean that electrons of all materials resonate or nearly resonate with red light's frequency? Shouldn't each material's refractive index be maximum for a different wavelength (frequency) and minimum for a different wavelength (frequency)?

 

[Meir Achuz]

" Does this mean that electrons of all materials resonate or nearly resonate with red light's frequency?"
Any resonance occurs OUTSIDE the visible region. If you look at the curve in the stack exchange post, you will see that the index n is higher for blue than red, whether the resonance is above or below the visible region.