permittivity

Refraction works because the permittivity of the transparent material is different from the permittivity of vacuum.

There's the concept of permittivity wherein Maxwell's equations are defined in free space with vacuum permittivity.

These universal constants (vacuum permittivity and permeability) are themselves dimensionful quantities, so the same comment applies to them.

Unfortunately negative refraction a secondary effect where you have negative permittivity and permeability at the same time.

In terms of photonic permittivity, Glass is better for cold frames and the like, because acrylic filters out UV light.

This prediction is dependent on assuming that the vacuum permeability and permittivity are constant numbers.

The two polymorphs of the material had a difference in some electrical property (permittivity?

The permeability/permittivity of the surrounding substance probably needs to be accounted for.

Not only that, but if you change the permittivity and permeability to that of any material, you get the speed of light in that material.

Every piece of matter has an associated electric permittivity and permeability.

(That actually is related to the speed of light in the insulator - they both depend on its permittivity).

Light is a alternating electric field and the speed of it's propagation is limited by the ε0 and μ0 (electric permittivity and magnetic permeability) in Maxwell's equations.

Two years ago in my E & M lab, one of my profs was showing off on of his research projects into materials with negative permeability and negative permittivity… it started to make me think about if people could have clothes(or objects) and spray a lining of these type of materials on them, it could render them "invisible" to traditional optics… Isn't the military doing research like this too?