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.
Here is a small table of dielectric constants:
| medium | ε/ε0 | refractive index |
| vacuum | 1.0000 | 1.0000 |
| air | 1.0006 | 1.0003 |
| paper | ||
| wood | ||
| rubber | ||
| dry soil | 3.9 | |
| moist soil | 29 | |
| water | 81 | 1.333 |