Dielectric constant or relative permittivity of a medium is the ratio of the electrostatic force between two charges separated by a certain distance in vacuum, to the electrostatic force between them at the same distance apart in the medium under consideration.
Dielectrics seem to make space apparently bigger or smaller than it actually is. The dielectric constant value tells you exactly how much smaller or bigger the space gets, on introduction of the medium.
`F=(q_1*q_2)/(4pi*epsilon_o*r^2)` (in vacuum)
and `F=(q_1*q_2)/(4pi*epsilon_o*epsilon*r^2)` (in the medium under consideration, of dielectric constant value, `epsilon` )
So, the distance between the two charges, r is replaced by ‘`sqrtepsilonr` ’, in the medium. The value of epsilon lies between 1 and 100 for most of the common materials.
The dielectric constant of a material also affects how electromagnetic waves move through the material. Just like the forces between charges, the dielectric warps the space between waves (i.e. between two successive crests, or between two successive troughs) to make it look a different size. As a result, normal molecular motion inside the bulk is modified when an electromagnetic wave is passed through it. This phenomenon is made use of in dielectric heating of bulk materials, by using a microwave to pass through a medium having relatively high dielectric constant.