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. ε_{0} denotes the permittivity of free space (the ideal vacuum). The dielectric constant is the ratio ε/ε_{0}
Dielectric constant is used when designing electronic circuits, capacitors, and dielectric waveguides.
Here is a small table of dielectric constants:
medium | ε/ε_{0} | 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 |