Ordinary dielectric transparent media will have a higher index for blue than for red. There are always some absorption bands in the infrared and in the ultra-violet.

In between two absorption bands, the index alwaysincreasestoward shorter wavelengths.This is normal dispersion.

In the absorption band itself, the index willdecreasetoward shorter wavelengths. This latter phenomenon is called “anomalous dispersion.” (or sometimes “resonant dispersion.”)

There is a close relationship between the absorption versus wavelength and the index versus wavelength. Mathematically, it is known as the Kramers-Kronig relationship, which means any two media with identical absorption bands will have identical dispersion relationships.

When current is flowing in a conductor the magnetic field works out to push most of the conduction to the outer surfaces of the conductor. So a hollow copper pipe works about as well as a solid copper conductor, at least for high-frequency AC.

One way around this is to use a bunch of insulated wires, randomly twisted together and only tied together at the ends. The magnetic fields roughly cancel each other out so each wire sort-of is its own skin-effect universe, and all the wires in parallel carry the current with less resistance than one solid wire. Some high-efficiency coils in some old radios used this kind of wire, called Litz wire.