An important property of an electrical insulator a.k.a a dielectric is its dielectric constant or relative permittivity.

The symbol ε is used to denote the electric permittivity of a medium. ε0 denotes the permittivity of free space (the ideal vacuum). The dielectric constant is the ratio ε/ε0

Dielectric constant is used when designing electronic circuits, capacitors, and dielectric waveguides.

insert relation to refractive index

Here is a small table of dielectric constants:
mediumε/ε0refractive index
vacuum 1.0000 1.0000
air 1.0006 1.0003
dry soil 3.9  
moist soil 29  
water 81 1.333